Why my wordpress does not convert to another language?

I downloaded ar.mo and ar.po to the languages folder of the wp-content folder in wordpress … and then I changed the line
define(‘WPLANG’, ”);
to
define(‘WPLANG’, ‘ar’); in the wp-config.php file

but only the (search0 word changed to arabic … nothing else ALTHOUGH I did this in the past and many things changed to Arabic , such as (see more) and (comments) and (by) , etc .. I know, of course, that the content itself will not be changed

so, what I am missing or doing wrong??

please help .. or if there is another way, tell me ??