Merge branch '6495-client-language-as-required' into 'develop'

Setting the language as a required field when creating a client

Closes #6495

See merge request ispconfig/ispconfig3!1719

Author: Till Brehm

interface/web/client/form/client.tform.php

@@ -231,6 +231,9 @@
 		'language' => array (
 			'datatype' => 'VARCHAR',
 			'formtype' => 'SELECT',
+			'validators' => array ( 0 => array ( 'type' => 'NOTEMPTY',
+					'errmsg'=> 'language_error_empty'),
+			),
 			'default' => $conf["language"],
 			'value'  => $language_list,
 			'separator' => '',