#525 and master merged - added translations

This one is "just" the same as #525... but it can be merged while #525 cannot. Indeed, there have been changes in translations files from both master and #525. I've solved that and added many translations. The few remaining not-translated keys are the ones I don't understand clearly.

It gives a 2-in-1 merge : descriptions in Style and Rule and translations everywhere. It may not be the best configuration, as it makes the PR a bit harder to understand. It's easier for me to do it this way this time... I hope it will be accepted anyway.