Fix language selector on preferences page (#2794)

app/views/settings/preferences/show.html.haml

@@ -5,7 +5,12 @@
   = render 'shared/error_messages', object: current_user
 
   .fields-group
-    = f.input :locale, collection: I18n.available_locales, wrapper: :with_label, include_blank: false, label_method: lambda { |locale| human_locale(locale) }
+    = f.input :locale,
+      collection: I18n.available_locales,
+      wrapper: :with_label,
+      include_blank: false,
+      label_method: lambda { |locale| human_locale(locale) },
+      selected: I18n.locale
 
     = f.input :allowed_languages,
       collection: I18n.available_locales,