FIX: Notifications shouldn't use user locale unless allow_user_locale is enabled

app/mailers/user_notifications.rb

@@ -20,10 +20,15 @@ class UserNotifications < ActionMailer::Base
   end
 
   def signup_after_approval(user, opts = {})
+    locale = user_locale(user)
+    tips = I18n.t('system_messages.usage_tips.text_body_template',
+                  base_url: Discourse.base_url,
+                  locale: locale)
+
     build_email(user.email,
                 template: 'user_notifications.signup_after_approval',
-                locale: user_locale(user),
-                new_user_tips: I18n.t('system_messages.usage_tips.text_body_template', base_url: Discourse.base_url, locale: locale))
+                locale: locale,
+                new_user_tips: tips)
   end
 
   def notify_old_email(user, opts = {})
@@ -320,7 +325,7 @@ class UserNotifications < ActionMailer::Base
   protected
 
   def user_locale(user)
-    (user.locale.present? && I18n.available_locales.include?(user.locale.to_sym)) ? user.locale : nil
+    user.effective_locale
   end
 
   def email_post_markdown(post, add_posted_by = false)

spec/mailers/user_notifications_spec.rb

@@ -802,12 +802,15 @@ describe UserNotifications do
 
   describe "notifications from template" do
 
-    context "user locale has been set" do
+    context "user locale is allowed" do
+      before do
+        SiteSetting.default_locale = "en"
+        SiteSetting.allow_user_locale = true
+      end
 
       %w(signup signup_after_approval confirm_old_email notify_old_email confirm_new_email
          forgot_password admin_login account_created).each do |mail_type|
         include_examples "notification derived from template" do
-          SiteSetting.default_locale = "en"
           let(:locale) { "fr" }
           let(:mail_type) { mail_type }
           it "sets the locale" do
@@ -817,29 +820,19 @@ describe UserNotifications do
       end
     end
 
-    context "user locale has not been set" do
+    context "user locale is not allowed" do
+      before do
+        SiteSetting.default_locale = "en"
+        SiteSetting.allow_user_locale = false
+      end
+
       %w(signup signup_after_approval notify_old_email confirm_old_email confirm_new_email
          forgot_password admin_login account_created).each do |mail_type|
         include_examples "notification derived from template" do
-          SiteSetting.default_locale = "en"
-          let(:locale) { nil }
+          let(:locale) { "fr" }
           let(:mail_type) { mail_type }
           it "sets the locale" do
-            expects_build_with(has_entry(:locale, nil))
-          end
-        end
-      end
-    end
-
-    context "user locale is an empty string" do
-      %w(signup signup_after_approval notify_old_email confirm_new_email confirm_old_email
-         forgot_password admin_login account_created).each do |mail_type|
-        include_examples "notification derived from template" do
-          SiteSetting.default_locale = "en"
-          let(:locale) { "" }
-          let(:mail_type) { mail_type }
-          it "sets the locale" do
-            expects_build_with(has_entry(:locale, nil))
+            expects_build_with(has_entry(:locale, "en"))
           end
         end
       end