Feature editor grid translations

This PR adds some French and German translations for the feature editor grid GeoExt.ux.

See https://github.com/camptocamp/cgxp/issues/433

No answer regarding this issue = do it by yourself (Don't get too used to that...)

Perhaps would it be good that @eleu gets through the German modifications.

As I'm only able to order an espresso in Italian, I did not do anything for this language...