Issue #1974040 by Gábor Hojtsy: Fixed When installing in an RTL language, it...

Issue #1974040 by Gábor Hojtsy: Fixed When installing in an RTL language, it should be RTL from profile selection onwards.

core/includes/install.core.inc

@@ -288,6 +288,14 @@ function install_begin_request(&$install_state) {
   // This must go after drupal_bootstrap(), which unsets globals!
   global $conf;
 
+  // If we have a language selected and it is not yet saved in the system
+  // (eg. pre-database data screens we are unable to persistently store
+  // the default language), we should set language_default so the proper
+  // language is used to display installer pages as early as possible.
+  if (!empty($install_state['parameters']['langcode']) && language_default()->langcode != $install_state['parameters']['langcode']) {
+    $GLOBALS['conf']['language_default'] = array('langcode' => $install_state['parameters']['langcode']);
+  }
+
   require_once DRUPAL_ROOT . '/core/modules/system/system.install';
   require_once DRUPAL_ROOT . '/core/includes/common.inc';
   require_once DRUPAL_ROOT . '/core/includes/file.inc';

core/includes/theme.inc

@@ -3099,9 +3099,16 @@ function template_preprocess_maintenance_page(&$variables) {
  * @see template_process_html()
  */
 function template_process_maintenance_page(&$variables) {
-  $variables['head']    = drupal_get_html_head();
-  $variables['css']     = drupal_add_css();
-  $variables['styles']  = drupal_get_css();
+  $variables['head'] = drupal_get_html_head();
+
+  // While this code is used in the installer, the language module may not be
+  // enabled yet (even maybe no database set up yet), but an RTL language
+  // selected should result in RTL stylesheets loaded properly already.
+  $variables['css'] = $css = drupal_add_css();
+  include_once DRUPAL_ROOT . '/core/modules/language/language.module';
+  language_css_alter($css);
+  $variables['styles'] = drupal_get_css($css);
+
   $variables['scripts'] = drupal_get_js();
 }