fix error, when only one language was selected

index.php

 
 // set language
 $languages = $f3->get('languages');
+if ( ! is_array($languages) ) {
+    $languages = [$languages];
+}
 $f3->set('default_lang', array_shift($languages));
 if (in_array(strtolower($f3->get('GET.lang')), $languages)) {
-  $f3->set('LANG', strtolower($f3->get('GET.lang')));
+    $f3->set('LANG', strtolower($f3->get('GET.lang')));
 } else {
-  $f3->set('LANG', $f3->get('default_lang'));
+    $f3->set('LANG', $f3->get('default_lang'));
 }
+
 // set content dir
 if(array_key_exists($f3->get('LANG'), $f3->get('content'))) {
   $content_dir=$f3->get('content.'.$f3->get('LANG'));