Fixes for Gallery 2 French translation in module 'core'

After years using G2, I finally fixed the fr.po for core module and submitted it to latest registered French translator of Gallery2 several weeks again. After review, he encoded the following tracker on SF.net for everybody to benefit from it, even if no G2 update is ever published again:
https://sourceforge.net/tracker/?func=detail&aid=2846783&group_id=7130&atid=582564

As G2 is using gettext, updating it it fairly easy.

Enjoy!