I assume that for everyone except Ike possibly, Gallant's fluent French will be translated as Yeerkish. Ike might initially hear it as French, but his "friend from the other side" will eventually convert it to English. As for why he's speaking French, that's because he's presumed to be from a point in time when French would've been the lingua franca. Now there are more than a few extra European countries on Gallant's Earth, but it'd be amusing if that fact was still true.