fix(config) Forbid setting of unavailable locale

issue #6051

include/class_Language.inc

@@ -127,6 +127,26 @@ class Language
     return al2gt($langs);
   }
 
+  /*!
+   * \brief Checks if a locale is available
+   *
+   * \param string $lang Language locale to use
+   */
+  public static function isAvailable (string $lang): bool
+  {
+    /* Store current locale */
+    $locale = setlocale(LC_ALL, 0);
+
+    /* Try to switch */
+    list ($language, , $char) = parse_gettext_lang($lang);
+    $langset = setlocale(LC_ALL, $lang, $language.'.'.$char);
+
+    /* Set current locale back */
+    setlocale(LC_ALL, $locale);
+
+    return ($langset !== FALSE);
+  }
+
   /*!
    * \brief Get the language for the user connecting
    *

plugins/config/class_configInLdap.inc

@@ -533,6 +533,9 @@ class configInLdap extends simplePlugin
     if (($this->fdPasswordDefaultHash == 'sasl') && ($this->fdSaslRealm == '') && ($this->fdSaslExop == '')) {
       $messages[] = _('You need to fill saslRealm or saslExop in the configuration screen in order to use SASL');
     }
+    if ($this->attributesAccess['fdLanguage']->hasChanged() && ($this->fdLanguage != '') && !Language::isAvailable($this->fdLanguage)) {
+      $messages[] = sprintf(_('It seems the selected language "%s" is not installed on the system. Please install it or select an other one.'), $this->fdLanguage);
+    }
     return $messages;
   }