Update fr_FR.lang

De nouvelle traduction à faire
To avoid redundancy, please don't duplicate the English strings into the localization file. This is particularly relevant for the usage info below, where typos are more likely due to the amount of text. I don't want to have to make changes in multiple files in that case. Missing strings automatically fall back to the English version anyway.

If someone else is also working on this translation, please coordinate some other way than via comments in the localization file. Otherwise just keep local notes / make a PR after you're done.