diff --git a/app/assets/javascripts/admin/addon/controllers/admin-site-text-edit.js b/app/assets/javascripts/admin/addon/controllers/admin-site-text-edit.js
index 99e1c19af3..cc7bac3267 100644
--- a/app/assets/javascripts/admin/addon/controllers/admin-site-text-edit.js
+++ b/app/assets/javascripts/admin/addon/controllers/admin-site-text-edit.js
@@ -7,6 +7,7 @@ import { popupAjaxError } from "discourse/lib/ajax-error";
 
 export default Controller.extend(bufferedProperty("siteText"), {
   saved: false,
+  queryParams: ["locale"],
 
   @discourseComputed("buffered.value")
   saveDisabled(value) {
@@ -15,9 +16,11 @@ export default Controller.extend(bufferedProperty("siteText"), {
 
   actions: {
     saveChanges() {
-      const buffered = this.buffered;
+      const attrs = this.buffered.getProperties("value");
+      attrs.locale = this.locale;
+
       this.siteText
-        .save(buffered.getProperties("value"))
+        .save(attrs)
         .then(() => {
           this.commitBuffer();
           this.set("saved", true);
@@ -27,10 +30,11 @@ export default Controller.extend(bufferedProperty("siteText"), {
 
     revertChanges() {
       this.set("saved", false);
+
       bootbox.confirm(I18n.t("admin.site_text.revert_confirm"), (result) => {
         if (result) {
           this.siteText
-            .revert()
+            .revert(this.locale)
             .then((props) => {
               const buffered = this.buffered;
               buffered.setProperties(props);
diff --git a/app/assets/javascripts/admin/addon/controllers/admin-site-text-index.js b/app/assets/javascripts/admin/addon/controllers/admin-site-text-index.js
index 289c49da87..3c0dc9c3b0 100644
--- a/app/assets/javascripts/admin/addon/controllers/admin-site-text-index.js
+++ b/app/assets/javascripts/admin/addon/controllers/admin-site-text-index.js
@@ -1,4 +1,5 @@
 import Controller from "@ember/controller";
+import discourseComputed from "discourse-common/utils/decorators";
 import discourseDebounce from "discourse-common/lib/debounce";
 let lastSearch;
 
@@ -6,23 +7,49 @@ export default Controller.extend({
   searching: false,
   siteTexts: null,
   preferred: false,
-  queryParams: ["q", "overridden"],
+  queryParams: ["q", "overridden", "locale"],
+  locale: null,
 
   q: null,
   overridden: false,
 
+  init() {
+    this._super(...arguments);
+
+    this.set("locale", this.siteSettings.default_locale);
+  },
+
   _performSearch() {
     this.store
-      .find("site-text", this.getProperties("q", "overridden"))
+      .find("site-text", this.getProperties("q", "overridden", "locale"))
       .then((results) => {
         this.set("siteTexts", results);
       })
       .finally(() => this.set("searching", false));
   },
 
+  @discourseComputed()
+  availableLocales() {
+    return JSON.parse(this.siteSettings.available_locales);
+  },
+
+  @discourseComputed("locale")
+  fallbackLocaleFullName() {
+    if (this.siteTexts.extras.fallback_locale) {
+      return this.availableLocales.find((l) => {
+        return l.value === this.siteTexts.extras.fallback_locale;
+      }).name;
+    }
+  },
+
   actions: {
     edit(siteText) {
-      this.transitionToRoute("adminSiteText.edit", siteText.get("id"));
+      this.transitionToRoute("adminSiteText.edit", siteText.get("id"), {
+        queryParams: {
+          locale: this.locale,
+          localeFullName: this.availableLocales[this.locale],
+        },
+      });
     },
 
     toggleOverridden() {
@@ -39,5 +66,14 @@ export default Controller.extend({
         lastSearch = q;
       }
     },
+
+    updateLocale(value) {
+      this.setProperties({
+        searching: true,
+        locale: value,
+      });
+
+      discourseDebounce(this, this._performSearch, 400);
+    },
   },
 });
diff --git a/app/assets/javascripts/admin/addon/models/site-text.js b/app/assets/javascripts/admin/addon/models/site-text.js
index d6c4c4d130..c25a5b30a0 100644
--- a/app/assets/javascripts/admin/addon/models/site-text.js
+++ b/app/assets/javascripts/admin/addon/models/site-text.js
@@ -1,10 +1,10 @@
 import RestModel from "discourse/models/rest";
 import { ajax } from "discourse/lib/ajax";
-const { getProperties } = Ember;
+import { getProperties } from "@ember/object";
 
 export default RestModel.extend({
-  revert() {
-    return ajax(`/admin/customize/site_texts/${this.id}`, {
+  revert(locale) {
+    return ajax(`/admin/customize/site_texts/${this.id}?locale=${locale}`, {
       type: "DELETE",
     }).then((result) => getProperties(result.site_text, "value", "can_revert"));
   },
diff --git a/app/assets/javascripts/admin/addon/routes/admin-site-text-edit.js b/app/assets/javascripts/admin/addon/routes/admin-site-text-edit.js
index fe720907c7..40c74fced3 100644
--- a/app/assets/javascripts/admin/addon/routes/admin-site-text-edit.js
+++ b/app/assets/javascripts/admin/addon/routes/admin-site-text-edit.js
@@ -1,10 +1,30 @@
 import Route from "@ember/routing/route";
+import { ajax } from "discourse/lib/ajax";
+
 export default Route.extend({
+  queryParams: {
+    locale: { replace: true },
+  },
+
   model(params) {
-    return this.store.find("site-text", params.id);
+    return ajax(
+      `/admin/customize/site_texts/${params.id}?locale=${params.locale}`
+    ).then((result) => {
+      return this.store.createRecord("site-text", result.site_text);
+    });
   },
 
   setupController(controller, siteText) {
-    controller.setProperties({ siteText, saved: false });
+    const locales = JSON.parse(this.siteSettings.available_locales);
+
+    const localeFullName = locales.find((locale) => {
+      return locale.value === controller.locale;
+    }).name;
+
+    controller.setProperties({
+      siteText,
+      saved: false,
+      localeFullName: localeFullName,
+    });
   },
 });
diff --git a/app/assets/javascripts/admin/addon/routes/admin-site-text-index.js b/app/assets/javascripts/admin/addon/routes/admin-site-text-index.js
index 85dfe8e8c1..255315a846 100644
--- a/app/assets/javascripts/admin/addon/routes/admin-site-text-index.js
+++ b/app/assets/javascripts/admin/addon/routes/admin-site-text-index.js
@@ -6,12 +6,13 @@ export default Route.extend({
   queryParams: {
     q: { replace: true },
     overridden: { replace: true },
+    locale: { replace: true },
   },
 
   model(params) {
     return this.store.find(
       "site-text",
-      getProperties(params, "q", "overridden")
+      getProperties(params, "q", "overridden", "locale")
     );
   },
 
diff --git a/app/assets/javascripts/admin/addon/templates/site-text-edit.hbs b/app/assets/javascripts/admin/addon/templates/site-text-edit.hbs
index 279a5a5acb..2bf960c24b 100644
--- a/app/assets/javascripts/admin/addon/templates/site-text-edit.hbs
+++ b/app/assets/javascripts/admin/addon/templates/site-text-edit.hbs
@@ -1,9 +1,12 @@
 <div class="edit-site-text">
-
   <div class="title">
     <h3>{{siteText.id}}</h3>
   </div>
 
+  <div class="title">
+    <h4>{{i18n "admin.site_text.locale"}} {{localeFullName}}</h4>
+  </div>
+
   {{expanding-text-area value=buffered.value rows="1" class="site-text-value"}}
 
   {{#save-controls model=siteText action=(action "saveChanges") saved=saved saveDisabled=saveDisabled}}
@@ -12,7 +15,7 @@
     {{/if}}
   {{/save-controls}}
 
-  {{#link-to "adminSiteText.index" class="go-back"}}
+  {{#link-to "adminSiteText.index" (query-params locale=locale) class="go-back"}}
     {{d-icon "arrow-left"}}
     {{i18n "admin.site_text.go_back"}}
   {{/link-to}}
diff --git a/app/assets/javascripts/admin/addon/templates/site-text-index.hbs b/app/assets/javascripts/admin/addon/templates/site-text-index.hbs
index fa72cc36f4..16e835d270 100644
--- a/app/assets/javascripts/admin/addon/templates/site-text-index.hbs
+++ b/app/assets/javascripts/admin/addon/templates/site-text-index.hbs
@@ -16,6 +16,21 @@
   </div>
 
   <p class="filter-options">
+    <div class="locale">
+      <label>{{i18n "admin.site_text.locale"}}</label>
+      {{combo-box
+        valueProperty="value"
+        content=availableLocales
+        value=locale
+        onChange=(action "updateLocale")
+        class="locale-search"
+        options=(hash
+          filterable=true
+          none="user.locale.default"
+        )
+      }}
+    </div>
+
     <label>
       {{input type="checkbox" checked=overridden click=(action "toggleOverridden")}}
       {{i18n "admin.site_text.show_overriden"}}
@@ -24,6 +39,13 @@
 </div>
 
 {{#conditional-loading-spinner condition=searching}}
+  {{#if fallbackLocaleFullName}}
+    <div class="alert alert-info">
+      {{d-icon "exclamation-circle"}}
+      {{i18n "admin.site_text.fallback_locale_warning" fallback=fallbackLocaleFullName}}
+    </div>
+  {{/if}}
+
   {{#if siteTexts.extras.recommended}}
     <p><b>{{i18n "admin.site_text.recommended"}}</b></p>
   {{/if}}
diff --git a/app/assets/javascripts/discourse/tests/acceptance/admin-site-text-test.js b/app/assets/javascripts/discourse/tests/acceptance/admin-site-text-test.js
index d477fd90c5..5cfcb3b7ec 100644
--- a/app/assets/javascripts/discourse/tests/acceptance/admin-site-text-test.js
+++ b/app/assets/javascripts/discourse/tests/acceptance/admin-site-text-test.js
@@ -8,6 +8,10 @@ import { test } from "qunit";
 
 acceptance("Admin - Site Texts", function (needs) {
   needs.user();
+  needs.settings({
+    available_locales: JSON.stringify([{ name: "English", value: "en" }]),
+    default_locale: "en",
+  });
 
   test("search for a key", async function (assert) {
     await visit("/admin/customize/site_texts");
@@ -31,7 +35,7 @@ acceptance("Admin - Site Texts", function (needs) {
   });
 
   test("edit and revert a site text by key", async function (assert) {
-    await visit("/admin/customize/site_texts/site.test");
+    await visit("/admin/customize/site_texts/site.test?locale=en");
 
     assert.equal(queryAll(".title h3").text(), "site.test");
     assert.ok(!exists(".saved"));
diff --git a/app/assets/javascripts/discourse/tests/helpers/create-pretender.js b/app/assets/javascripts/discourse/tests/helpers/create-pretender.js
index 288e8375e1..b75d641d33 100644
--- a/app/assets/javascripts/discourse/tests/helpers/create-pretender.js
+++ b/app/assets/javascripts/discourse/tests/helpers/create-pretender.js
@@ -644,9 +644,15 @@ export function applyDefaultHandlers(pretender) {
 
   pretender.get("/admin/customize/site_texts", (request) => {
     if (request.queryParams.overridden) {
-      return response(200, { site_texts: [overridden] });
+      return response(200, {
+        site_texts: [overridden],
+        extras: { locale: "en" },
+      });
     } else {
-      return response(200, { site_texts: [siteText, overridden] });
+      return response(200, {
+        site_texts: [siteText, overridden],
+        extras: { locale: "en" },
+      });
     }
   });
 
diff --git a/app/assets/stylesheets/common/admin/admin_base.scss b/app/assets/stylesheets/common/admin/admin_base.scss
index f17be2880e..a65d7ff0ad 100644
--- a/app/assets/stylesheets/common/admin/admin_base.scss
+++ b/app/assets/stylesheets/common/admin/admin_base.scss
@@ -204,6 +204,12 @@ $mobile-breakpoint: 700px;
     .reseed {
       float: right;
     }
+    .locale {
+      margin-bottom: 0.5em;
+    }
+    .locale-search {
+      width: 50%;
+    }
   }
   .text-highlight {
     font-weight: bold;
diff --git a/app/controllers/admin/site_texts_controller.rb b/app/controllers/admin/site_texts_controller.rb
index 4f0226bbb1..ca13e68895 100644
--- a/app/controllers/admin/site_texts_controller.rb
+++ b/app/controllers/admin/site_texts_controller.rb
@@ -21,14 +21,13 @@ class Admin::SiteTextsController < Admin::AdminController
 
     query = params[:q] || ""
 
-    locale = params[:locale] || I18n.locale
-    raise Discourse::InvalidParameters.new(:locale) if !I18n.locale_available?(locale)
+    locale = fetch_locale(params[:locale])
 
     if query.blank? && !overridden
       extras[:recommended] = true
-      results = I18n.with_locale(locale) { self.class.preferred_keys.map { |k| record_for(k) } }
+      results = self.class.preferred_keys.map { |k| record_for(key: k, locale: locale) }
     else
-      results = I18n.with_locale(locale) { find_translations(query, overridden) }
+      results = find_translations(query, overridden, locale)
 
       if results.any?
         extras[:regex] = I18n::Backend::DiscourseI18n.create_search_regexp(query, as_string: true)
@@ -53,20 +52,37 @@ class Admin::SiteTextsController < Admin::AdminController
     last = first + per_page
 
     extras[:has_more] = true if results.size > last
-    render_serialized(results[first..last - 1], SiteTextSerializer, root: 'site_texts', rest_serializer: true, extras: extras, overridden_keys: overridden_keys)
+
+    if LocaleSiteSetting.fallback_locale(locale).present?
+      extras[:fallback_locale] = LocaleSiteSetting.fallback_locale(locale)
+    end
+
+    overridden = overridden_keys(locale)
+    render_serialized(
+      results[first..last - 1],
+      SiteTextSerializer,
+      root: 'site_texts',
+      rest_serializer: true,
+      extras: extras,
+      overridden_keys: overridden,
+    )
   end
 
   def show
-    site_text = find_site_text
+    locale = fetch_locale(params[:locale])
+    site_text = find_site_text(locale)
     render_serialized(site_text, SiteTextSerializer, root: 'site_text', rest_serializer: true)
   end
 
   def update
-    site_text = find_site_text
-    value = site_text[:value] = params[:site_text][:value]
+    locale = fetch_locale(params.dig(:site_text, :locale))
+
+    site_text = find_site_text(locale)
+    value = site_text[:value] = params.dig(:site_text, :value)
     id = site_text[:id]
-    old_value = I18n.t(id)
-    translation_override = TranslationOverride.upsert!(I18n.locale, id, value)
+    old_value = I18n.t(id, locale: locale)
+
+    translation_override = TranslationOverride.upsert!(locale, id, value)
 
     if translation_override.errors.empty?
       StaffActionLogger.new(current_user).log_site_text_change(id, value, old_value)
@@ -88,11 +104,14 @@ class Admin::SiteTextsController < Admin::AdminController
   end
 
   def revert
-    site_text = find_site_text
+    locale = fetch_locale(params[:locale])
+
+    site_text = find_site_text(locale)
     id = site_text[:id]
-    old_text = I18n.t(id)
-    TranslationOverride.revert!(I18n.locale, id)
-    site_text = find_site_text
+    old_text = I18n.t(id, locale: locale)
+    TranslationOverride.revert!(locale, id)
+
+    site_text = find_site_text(locale)
     StaffActionLogger.new(current_user).log_site_text_change(id, site_text[:value], old_text)
     system_badge_id = Badge.find_system_badge_id_from_translation_key(id)
     if system_badge_id.present?
@@ -137,39 +156,39 @@ class Admin::SiteTextsController < Admin::AdminController
     badge_parts[0] == 'badges' && badge_parts[2] == 'name'
   end
 
-  def record_for(key, value = nil)
+  def record_for(key:, value: nil, locale:)
     if key.ends_with?("_MF")
-      override = TranslationOverride.where(translation_key: key, locale: I18n.locale).pluck(:value)
+      override = TranslationOverride.where(translation_key: key, locale: locale).pluck(:value)
       value = override&.first
     end
 
-    value ||= I18n.t(key)
+    value ||= I18n.t(key, locale: locale)
     { id: key, value: value }
   end
 
   PLURALIZED_REGEX = /(.*)\.(zero|one|two|few|many|other)$/
 
-  def find_site_text
+  def find_site_text(locale)
     if self.class.restricted_keys.include?(params[:id])
       raise Discourse::InvalidAccess.new(nil, nil, custom_message: 'email_template_cant_be_modified')
     end
 
-    if I18n.exists?(params[:id]) || TranslationOverride.exists?(locale: I18n.locale, translation_key: params[:id])
-      return record_for(params[:id])
+    if I18n.exists?(params[:id], locale) || TranslationOverride.exists?(locale: locale, translation_key: params[:id])
+      return record_for(key: params[:id], locale: locale)
     end
 
     if PLURALIZED_REGEX.match(params[:id])
-      value = fix_plural_keys($1, {}).fetch($2.to_sym)
-      return record_for(params[:id], value) if value
+      value = fix_plural_keys($1, {}, locale).detect { |plural| plural[0] == $2.to_sym }
+      return record_for(key: params[:id], value: value[1], locale: value[2]) if value
     end
 
     raise Discourse::NotFound
   end
 
-  def find_translations(query, overridden)
+  def find_translations(query, overridden, locale)
     translations = Hash.new { |hash, key| hash[key] = {} }
 
-    I18n.search(query, overridden: overridden).each do |key, value|
+    I18n.search(query, overridden: overridden, locale: locale).each do |key, value|
       if PLURALIZED_REGEX.match(key)
         translations[$1][$2] = value
       else
@@ -183,30 +202,46 @@ class Admin::SiteTextsController < Admin::AdminController
       next unless I18n.exists?(key, :en)
 
       if value&.is_a?(Hash)
-        value = fix_plural_keys(key, value)
-        value.each do |plural_key, plural_value|
-          results << record_for("#{key}.#{plural_key}", plural_value)
+        fix_plural_keys(key, value, locale).each do |plural|
+          plural_key = plural[0]
+          plural_value = plural[1]
+
+          results << record_for(
+            key: "#{key}.#{plural_key}", value: plural_value, locale: plural.last
+          )
         end
       else
-        results << record_for(key, value)
+        results << record_for(key: key, value: value, locale: locale)
       end
     end
 
     results
   end
 
-  def fix_plural_keys(key, value)
+  def fix_plural_keys(key, value, locale)
     value = value.with_indifferent_access
-    plural_keys = I18n.t('i18n.plural.keys')
+    plural_keys = I18n.t('i18n.plural.keys', locale: locale)
     return value if value.keys.size == plural_keys.size && plural_keys.all? { |k| value.key?(k) }
 
     fallback_value = I18n.t(key, locale: :en, default: {})
     plural_keys.map do |k|
-      [k, value[k] || fallback_value[k] || fallback_value[:other]]
-    end.to_h
+      if value[k]
+        [k, value[k], locale]
+      else
+        [k, fallback_value[k] || fallback_value[:other], :en]
+      end
+    end
   end
 
-  def overridden_keys
-    TranslationOverride.where(locale: I18n.locale).pluck(:translation_key)
+  def overridden_keys(locale)
+    TranslationOverride.where(locale: locale).pluck(:translation_key)
+  end
+
+  def fetch_locale(locale_from_params)
+    locale_from_params.tap do |locale|
+      if locale.blank? || !I18n.locale_available?(locale)
+        raise Discourse::InvalidParameters.new(:locale)
+      end
+    end
   end
 end
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index 67818f6503..3449b996d8 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -4826,6 +4826,8 @@ en:
         go_back: "Back to Search"
         recommended: "We recommend customizing the following text to suit your needs:"
         show_overriden: "Only show overridden"
+        locale: "Language:"
+        fallback_locale_warning: "You are editing a variety of %{fallback}. Users who choose %{fallback} as their interface language won't see your changes."
         more_than_50_results: "There are more than 50 results. Please refine your search."
 
       settings: # used by theme and site settings
diff --git a/lib/freedom_patches/translate_accelerator.rb b/lib/freedom_patches/translate_accelerator.rb
index 5a82623f8f..5d973ddd91 100644
--- a/lib/freedom_patches/translate_accelerator.rb
+++ b/lib/freedom_patches/translate_accelerator.rb
@@ -71,7 +71,7 @@ module I18n
       opts ||= {}
 
       target = opts[:backend] || backend
-      results = opts[:overridden] ? {} : target.search(config.locale, query)
+      results = opts[:overridden] ? {} : target.search(locale, query)
 
       regexp = I18n::Backend::DiscourseI18n.create_search_regexp(query)
       (overrides_by_locale(locale) || {}).each do |k, v|
diff --git a/spec/requests/admin/site_texts_controller_spec.rb b/spec/requests/admin/site_texts_controller_spec.rb
index 09985c0076..63d9f5349f 100644
--- a/spec/requests/admin/site_texts_controller_spec.rb
+++ b/spec/requests/admin/site_texts_controller_spec.rb
@@ -5,6 +5,7 @@ require 'rails_helper'
 RSpec.describe Admin::SiteTextsController do
   fab!(:admin) { Fabricate(:admin) }
   fab!(:user) { Fabricate(:user) }
+  let(:default_locale) { I18n.locale }
 
   after do
     TranslationOverride.delete_all
@@ -18,7 +19,7 @@ RSpec.describe Admin::SiteTextsController do
   context "when not logged in as an admin" do
     it "raises an error if you aren't logged in" do
       put '/admin/customize/site_texts/some_key.json', params: {
-        site_text: { value: 'foo' }
+        site_text: { value: 'foo' }, locale: default_locale
       }
 
       expect(response.status).to eq(404)
@@ -28,7 +29,7 @@ RSpec.describe Admin::SiteTextsController do
       sign_in(user)
 
       put "/admin/customize/site_texts/some_key.json", params: {
-        site_text: { value: 'foo' }
+        site_text: { value: 'foo' }, locale: default_locale
       }
       expect(response.status).to eq(404)
 
@@ -46,13 +47,13 @@ RSpec.describe Admin::SiteTextsController do
 
     describe '#index' do
       it 'returns json' do
-        get "/admin/customize/site_texts.json", params: { q: 'title' }
+        get "/admin/customize/site_texts.json", params: { q: 'title', locale: default_locale }
         expect(response.status).to eq(200)
         expect(response.parsed_body['site_texts']).to include(include("id" => "title"))
       end
 
       it 'sets has_more to true if more than 50 results were found' do
-        get "/admin/customize/site_texts.json", params: { q: 'e' }
+        get "/admin/customize/site_texts.json", params: { q: 'e', locale: default_locale }
         expect(response.status).to eq(200)
         expect(response.parsed_body['site_texts'].size).to eq(50)
         expect(response.parsed_body['extras']['has_more']).to be_truthy
@@ -61,23 +62,23 @@ RSpec.describe Admin::SiteTextsController do
       it 'works with pages' do
         texts = Set.new
 
-        get "/admin/customize/site_texts.json", params: { q: 'e' }
+        get "/admin/customize/site_texts.json", params: { q: 'e', locale: default_locale }
         response.parsed_body['site_texts'].each { |text| texts << text['id'] }
         expect(texts.size).to eq(50)
 
-        get "/admin/customize/site_texts.json", params: { q: 'e', page: 1 }
+        get "/admin/customize/site_texts.json", params: { q: 'e', page: 1, locale: default_locale }
         response.parsed_body['site_texts'].each { |text| texts << text['id'] }
         expect(texts.size).to eq(100)
       end
 
       it 'works with locales' do
-        get "/admin/customize/site_texts.json", params: { q: 'yes_value', locale: 'en' }
+        get "/admin/customize/site_texts.json", params: { q: 'yes_value', locale: default_locale }
         value = response.parsed_body['site_texts'].find { |text| text['id'] == 'js.yes_value' }['value']
-        expect(value).to eq(I18n.with_locale(:en) { I18n.t('js.yes_value') })
+        expect(value).to eq(I18n.t('js.yes_value', locale: default_locale))
 
         get "/admin/customize/site_texts.json", params: { q: 'yes_value', locale: 'de' }
         value = response.parsed_body['site_texts'].find { |text| text['id'] == 'js.yes_value' }['value']
-        expect(value).to eq(I18n.with_locale(:de) { I18n.t('js.yes_value') })
+        expect(value).to eq(I18n.t('js.yes_value', locale: :de))
       end
 
       it 'returns an error on invalid locale' do
@@ -85,9 +86,14 @@ RSpec.describe Admin::SiteTextsController do
         expect(response.status).to eq(400)
       end
 
+      it 'returns an error on empty locale' do
+        get "/admin/customize/site_texts.json"
+        expect(response.status).to eq(400)
+      end
+
       it 'normalizes quotes during search' do
         value = %q|“That’s a ‘magic’ sock.”|
-        put "/admin/customize/site_texts/title.json", params: { site_text: { value: value } }
+        put "/admin/customize/site_texts/title.json", params: { site_text: { value: value, locale: default_locale } }
 
         [
           %q|That's a 'magic' sock.|,
@@ -97,7 +103,7 @@ RSpec.describe Admin::SiteTextsController do
           %q|«That's a 'magic' sock.»|,
           %q|„That’s a ‚magic‘ sock.“|
         ].each do |search_term|
-          get "/admin/customize/site_texts.json", params: { q: search_term }
+          get "/admin/customize/site_texts.json", params: { q: search_term, locale: default_locale }
           expect(response.status).to eq(200)
           expect(response.parsed_body['site_texts']).to include(include("id" => "title", "value" => value))
         end
@@ -105,14 +111,14 @@ RSpec.describe Admin::SiteTextsController do
 
       it 'normalizes ellipsis' do
         value = "Loading Discussion…"
-        put "/admin/customize/site_texts/embed.loading.json", params: { site_text: { value: value } }
+        put "/admin/customize/site_texts/embed.loading.json", params: { site_text: { value: value, locale: default_locale } }
 
         [
           "Loading Discussion",
           "Loading Discussion...",
           "Loading Discussion…"
         ].each do |search_term|
-          get "/admin/customize/site_texts.json", params: { q: search_term }
+          get "/admin/customize/site_texts.json", params: { q: search_term, locale: default_locale }
           expect(response.status).to eq(200)
           expect(response.parsed_body['site_texts']).to include(include("id" => "embed.loading", "value" => value))
         end
@@ -123,11 +129,11 @@ RSpec.describe Admin::SiteTextsController do
         TranslationOverride.create!(locale: :ru, translation_key: 'missing_plural_key.one', value: 'ONE')
         TranslationOverride.create!(locale: :ru, translation_key: 'another_missing_key', value: 'foo')
 
-        get "/admin/customize/site_texts.json", params: { q: 'missing_plural_key' }
+        get "/admin/customize/site_texts.json", params: { q: 'missing_plural_key', locale: default_locale }
         expect(response.status).to eq(200)
         expect(response.parsed_body['site_texts']).to be_empty
 
-        get "/admin/customize/site_texts.json", params: { q: 'another_missing_key' }
+        get "/admin/customize/site_texts.json", params: { q: 'another_missing_key', locale: default_locale }
         expect(response.status).to eq(200)
         expect(response.parsed_body['site_texts']).to be_empty
       end
@@ -141,7 +147,7 @@ RSpec.describe Admin::SiteTextsController do
           it 'finds the correct plural keys for the locale' do
             SiteSetting.default_locale = locale
 
-            get '/admin/customize/site_texts.json', params: { q: 'colour' }
+            get '/admin/customize/site_texts.json', params: { q: 'colour', locale: locale }
             expect(response.status).to eq(200)
 
             json = ::JSON.parse(response.body, symbolize_names: true)
@@ -202,7 +208,7 @@ RSpec.describe Admin::SiteTextsController do
 
     describe '#show' do
       it 'returns a site text for a key that exists' do
-        get "/admin/customize/site_texts/js.topic.list.json"
+        get "/admin/customize/site_texts/js.topic.list.json", params: { locale: default_locale }
         expect(response.status).to eq(200)
 
         json = response.parsed_body
@@ -214,7 +220,7 @@ RSpec.describe Admin::SiteTextsController do
       end
 
       it 'returns a site text for a key with ampersand' do
-        get "/admin/customize/site_texts/js.emoji_picker.food_&_drink.json"
+        get "/admin/customize/site_texts/js.emoji_picker.food_&_drink.json", params: { locale: default_locale }
         expect(response.status).to eq(200)
 
         json = response.parsed_body
@@ -226,30 +232,46 @@ RSpec.describe Admin::SiteTextsController do
       end
 
       it 'returns not found for missing keys' do
-        get "/admin/customize/site_texts/made_up_no_key_exists.json"
+        get "/admin/customize/site_texts/made_up_no_key_exists.json", params: { locale: default_locale }
         expect(response.status).to eq(404)
       end
 
       it 'returns overridden = true if there is a translation_overrides record for the key' do
         key = 'js.topic.list'
         put "/admin/customize/site_texts/#{key}.json", params: {
-          site_text: { value: I18n.t(key) }
+          site_text: { value: I18n.t(key, locale: default_locale), locale: default_locale }
         }
         expect(response.status).to eq(200)
 
-        get "/admin/customize/site_texts/#{key}.json"
+        get "/admin/customize/site_texts/#{key}.json", params: { locale: default_locale }
         expect(response.status).to eq(200)
         json = response.parsed_body
         expect(json['site_text']['overridden']).to eq(true)
 
         TranslationOverride.destroy_all
 
-        get "/admin/customize/site_texts/#{key}.json"
+        get "/admin/customize/site_texts/#{key}.json", params: { locale: default_locale }
         expect(response.status).to eq(200)
         json = response.parsed_body
         expect(json['site_text']['overridden']).to eq(false)
       end
 
+      it 'returns a site text in the given locale' do
+        get "/admin/customize/site_texts/js.topic.list.json", params: { locale: 'es' }
+        expect(response.status).to eq(200)
+
+        json = response.parsed_body
+        site_text = json['site_text']
+
+        expect(site_text['id']).to eq('js.topic.list')
+        expect(site_text['value']).to eq(I18n.t("js.topic.list", locale: :es))
+      end
+
+      it 'fails if locale is not given' do
+        get "/admin/customize/site_texts/js.topic.list.json"
+        expect(response.status).to eq(400)
+      end
+
       context 'plural keys' do
         before do
           I18n.backend.store_translations(:en, colour: { one: '%{count} colour', other: '%{count} colours' })
@@ -257,12 +279,10 @@ RSpec.describe Admin::SiteTextsController do
 
         shared_examples 'has correct plural keys' do
           it 'returns the correct plural keys for the locale' do
-            SiteSetting.default_locale = locale
-
             expected_translations.each do |key, value|
               id = "colour.#{key}"
 
-              get "/admin/customize/site_texts/#{id}.json"
+              get "/admin/customize/site_texts/#{id}.json", params: { locale: locale }
               expect(response.status).to eq(200)
 
               json = response.parsed_body
@@ -307,7 +327,7 @@ RSpec.describe Admin::SiteTextsController do
     describe '#update & #revert' do
       it "returns 'not found' when an unknown key is used" do
         put '/admin/customize/site_texts/some_key.json', params: {
-          site_text: { value: 'foo' }
+          site_text: { value: 'foo', locale: default_locale }
         }
 
         expect(response.status).to eq(404)
@@ -318,7 +338,7 @@ RSpec.describe Admin::SiteTextsController do
 
       it "works as expected with correct keys" do
         put '/admin/customize/site_texts/js.emoji_picker.animals_%26_nature.json', params: {
-          site_text: { value: 'foo' }
+          site_text: { value: 'foo', locale: default_locale }
         }
 
         expect(response.status).to eq(200)
@@ -332,7 +352,7 @@ RSpec.describe Admin::SiteTextsController do
 
       it "does not update restricted keys" do
         put '/admin/customize/site_texts/user_notifications.confirm_old_email.title.json', params: {
-          site_text: { value: 'foo' }
+          site_text: { value: 'foo', locale: default_locale }
         }
 
         expect(response.status).to eq(403)
@@ -347,7 +367,7 @@ RSpec.describe Admin::SiteTextsController do
         I18n.backend.store_translations(SiteSetting.default_locale, some_key: '%{first} %{second}')
 
         put "/admin/customize/site_texts/some_key.json", params: {
-          site_text: { value: 'hello %{key} %{omg}' }
+          site_text: { value: 'hello %{key} %{omg}', locale: default_locale }
         }
 
         expect(response.status).to eq(422)
@@ -364,7 +384,7 @@ RSpec.describe Admin::SiteTextsController do
         original_title = I18n.t(:title)
 
         put "/admin/customize/site_texts/title.json", params: {
-          site_text: { value: 'yay' }
+          site_text: { value: 'yay', locale: default_locale }
         }
         expect(response.status).to eq(200)
 
@@ -374,7 +394,7 @@ RSpec.describe Admin::SiteTextsController do
         expect(log.new_value).to eq('yay')
         expect(log.action).to eq(UserHistory.actions[:change_site_text])
 
-        delete "/admin/customize/site_texts/title.json"
+        delete "/admin/customize/site_texts/title.json", params: { locale: default_locale }
         expect(response.status).to eq(200)
 
         log = UserHistory.last
@@ -387,7 +407,7 @@ RSpec.describe Admin::SiteTextsController do
       it 'updates and reverts the key' do
         orig_title = I18n.t(:title)
 
-        put "/admin/customize/site_texts/title.json", params: { site_text: { value: 'hello' } }
+        put "/admin/customize/site_texts/title.json", params: { site_text: { value: 'hello', locale: default_locale } }
         expect(response.status).to eq(200)
 
         json = response.parsed_body
@@ -398,7 +418,7 @@ RSpec.describe Admin::SiteTextsController do
         expect(site_text['value']).to eq('hello')
 
         # Revert
-        delete "/admin/customize/site_texts/title.json"
+        delete "/admin/customize/site_texts/title.json", params: { locale: default_locale }
         expect(response.status).to eq(200)
 
         json = response.parsed_body
@@ -410,34 +430,34 @@ RSpec.describe Admin::SiteTextsController do
       end
 
       it 'returns site texts for the correct locale' do
-        SiteSetting.default_locale = :ru
+        locale = :ru
 
         ru_title = 'title ru'
         ru_mf_text = 'ru {NUM_RESULTS, plural, one {1 result} other {many} }'
 
-        put "/admin/customize/site_texts/title.json", params: { site_text: { value: ru_title } }
+        put "/admin/customize/site_texts/title.json", params: { site_text: { value: ru_title, locale: locale } }
         expect(response.status).to eq(200)
-        put "/admin/customize/site_texts/js.topic.read_more_MF.json", params: { site_text: { value: ru_mf_text } }
+        put "/admin/customize/site_texts/js.topic.read_more_MF.json", params: { site_text: { value: ru_mf_text, locale: locale } }
         expect(response.status).to eq(200)
 
-        get "/admin/customize/site_texts/title.json"
+        get "/admin/customize/site_texts/title.json", params: { locale: locale }
         expect(response.status).to eq(200)
         json = response.parsed_body
         expect(json['site_text']['value']).to eq(ru_title)
 
-        get "/admin/customize/site_texts/js.topic.read_more_MF.json"
+        get "/admin/customize/site_texts/js.topic.read_more_MF.json", params: { locale: locale }
         expect(response.status).to eq(200)
         json = response.parsed_body
         expect(json['site_text']['value']).to eq(ru_mf_text)
 
-        SiteSetting.default_locale = :en
+        en_locale = :en
 
-        get "/admin/customize/site_texts/title.json"
+        get "/admin/customize/site_texts/title.json", params: { locale: en_locale }
         expect(response.status).to eq(200)
         json = response.parsed_body
         expect(json['site_text']['value']).to_not eq(ru_title)
 
-        get "/admin/customize/site_texts/js.topic.read_more_MF.json"
+        get "/admin/customize/site_texts/js.topic.read_more_MF.json", params: { locale: en_locale }
         expect(response.status).to eq(200)
         json = response.parsed_body
         expect(json['site_text']['value']).to_not eq(ru_mf_text)
@@ -459,7 +479,7 @@ RSpec.describe Admin::SiteTextsController do
             action: Jobs::BulkUserTitleUpdate::UPDATE_ACTION
           }) do
             put '/admin/customize/site_texts/badges.regular.name.json', params: {
-              site_text: { value: 'Terminator' }
+              site_text: { value: 'Terminator', locale: default_locale }
             }
           end
 
@@ -468,14 +488,14 @@ RSpec.describe Admin::SiteTextsController do
             granted_badge_id: badge.id,
             action: Jobs::BulkUserTitleUpdate::RESET_ACTION
           }) do
-            delete "/admin/customize/site_texts/badges.regular.name.json"
+            delete "/admin/customize/site_texts/badges.regular.name.json", params: { locale: default_locale }
           end
         end
 
         it 'does not update matching user titles when overriding non-title badge text' do
           expect_not_enqueued_with(job: :bulk_user_title_update) do
             put '/admin/customize/site_texts/badges.regular.long_description.json', params: {
-              site_text: { value: 'Terminator' }
+              site_text: { value: 'Terminator', locale: default_locale }
             }
           end
         end