i tried a self-made hebrew translation for a a commercial plugin and a professional translation for a public plugin. in both cases the translation did not work . PLEASE see the attached image for example of implementation ! better image view you can see here: http://www.siz.co.il/my.php?i=rzm0mdtmyiqz.jpg
" Google translate' it is translate automatically and not as PO&MO files used in the translation of plugins and themes and the core wordpress system .
Make sure that the .mo file is in the /wp-content/languages directory. (If it doesn't exists, create it).