On Facebook, click your name on the top right to see your profile.
Click the 'About' link.
This should lead you to a page that looks like
facebook.com/<your name>/info
So, for example, mine is:
http://www.facebook.com/laurence.moroney/info
Find the 'Basic Info' section, and click 'Edit'.
If 'French' is listed as a language that you understand -- remove it.
When you tell facebook that you understand French, it doesn't give you the option to translate from French, because you have told them you already understand it.