Fix missing localization of HTML editor when assets_dir != INSTALL_PATH

CHANGELOG

@@ -26,6 +26,7 @@ CHANGELOG Roundcube Webmail
 - Don't create multipart/alternative messages with empty text/plain part (#5283)
 - Use contact_search_name format in popup on results in compose contacts search
 - Fix handling of 'mailto' and 'error' arguments in message_before_send hook (#5347)
+- Fix missing localization of HTML editor when assets_dir != INSTALL_PATH
 
 RELEASE 1.2.0
 -------------

program/include/rcmail.php

@@ -1917,13 +1917,14 @@ class rcmail extends rcube
         }
 
         $lang_codes = array($_SESSION['language']);
+        $assets_dir = $this->config->get('assets_dir') ?: INSTALL_PATH;
 
         if ($pos = strpos($_SESSION['language'], '_')) {
             $lang_codes[] = substr($_SESSION['language'], 0, $pos);
         }
 
         foreach ($lang_codes as $code) {
-            if (file_exists(INSTALL_PATH . 'program/js/tinymce/langs/'.$code.'.js')) {
+            if (file_exists("$assets_dir/program/js/tinymce/langs/$code.js")) {
                 $lang = $code;
                 break;
             }