French translation overhaul

[A-Printer]

While setting up my own private instance I noticed some strings were missing from the French translation. So I added them. I ended up reworking a lot of existing strings for consistency accross the app and fixed a few mistakes in the process.

I also noticed one string was missing from the English translation, so I added it.

Many thanks to the LinkStack contributors and to the original author of the French translation, @cedriclocqueneux !

PS: I guess I should also port the changes to the beta branch?