When adding a new language I notice that there are three options for the greek language but none of them has the correct locale. WordPress locale for greek is "el" (not "el_EL"). Take a look at http://wordpress.org/support/topic/problems-with-greek-locale-of-polylang-tranlation-og-blogs-titlesubtitles?replies=9 and http://codex.wordpress.org/WordPress_in_Your_Language#Greek_.28el.29
One more issue is that even after renaming the generated .mo file (to el.mo) the translated strings are not showing when switching languages from WPML switcher.
Any help is greatly appreciated!