Add a site setting to allow users to toggle I18n.locale

It is false by default.

app/assets/javascripts/discourse/controllers/preferences_controller.js

@@ -11,6 +11,10 @@ Discourse.PreferencesController = Discourse.ObjectController.extend({
     return Discourse.SiteSettings.allow_uploaded_avatars;
   }.property(),
 
+  allowUserLocale: function() {
+    return Discourse.SiteSettings.allow_user_locale;
+  }.property(),
+
   // By default we haven't saved anything
   saved: false,
 

app/assets/javascripts/discourse/templates/user/preferences.js.handlebars

@@ -66,15 +66,17 @@
       </div>
     </div>
 
-    <div class="control-group">
-      <label class="control-label">{{i18n user.locale.title}}</label>
-      <div class="controls">
-        {{combobox valueAttribute="value" content=availableLocales value=locale none="user.locale.default"}}
+    {{#if allowUserLocale}}
+      <div class="control-group">
+        <label class="control-label">{{i18n user.locale.title}}</label>
+        <div class="controls">
+          {{combobox valueAttribute="value" content=availableLocales value=locale none="user.locale.default"}}
+        </div>
+        <div class='instructions'>
+          {{i18n user.locale.instructions}}
+        </div>
       </div>
-      <div class='instructions'>
-        {{i18n user.locale.instructions}}
-      </div>
-    </div>
+    {{/if}}
 
     <div class="control-group">
       <label class="control-label">{{i18n user.bio}}</label>

app/controllers/application_controller.rb

@@ -109,7 +109,7 @@ class ApplicationController < ActionController::Base
   end
 
   def set_locale
-    I18n.locale = if current_user && current_user.locale.present?
+    I18n.locale = if SiteSetting.allow_user_locale && current_user && current_user.locale.present?
                     current_user.locale
                   else
                     SiteSetting.default_locale