Language

Hi there, I’m trying to enable site in my language – I translated .po file in POEDIT, uploaded it to /language (/wp-content/languages) but without effects. Adding define (.WPLANG., .pl_PL.) => internal 500 error of my server. I’d like to mention that in wp-config.php file already contains define (‘WPLANG’, ‘pl_PL). I tired to comment it and leave only define (.WPLANG., .pl_PL.) but no effects -> error 500 occured. Please help.
Regards