@Sh:
"works... set the language into English.. when I set it back to French, it fails again..."
There is your problem cause ;)
There must  "something" in the fr lang file that is causing - maybe encoding issues.
Upload your fr lang file someplace i can grab and i will take a quick look.

@rjcalifornia:

I change language in the setting page (where you have account name settings, account password, email settings and language settings).

The group plugin use this file plugin too.

@DhrupDeScoop:

The fr lang file is the default one (encoding UTF8 without BOM) : http://sexiicookies.free.fr/Tung/fr.php

Thanks for your help guys

I found the solution : the problem was the file '/elgg/languages/fr.php' encoded in UTF-8. We just have to convert it into UTF-8 without BOM.

To do this, you can use Notepad ++ for example.