Allow to modify qtranxf_isMultilingual function

[Buildbetter]

Please allow us to modify qtranxf_isMultilingual with add_filter, it will be very useful, i need to fix function because i get error when i use qtranslate-xt with "Custom Field Suite" plugin

I get Warning: preg_match() expects parameter 2 to be string, array given in /home/inannabg/zzzzzzz.com/wp-content/plugins/qtranslate-xt/qtranslate_utils.php on line 389