Historical Hetalia is fine so long as it's not an alt history; it's just a different, pre-canon canon point, really. Headcanon's all good to flesh things out. For the history section, feel free to link something akin to Wikipedia's History of France article rather than something specific to Hetalia for better context on everything. (One of the mods used to play Austria in 2011; they understand the Hetalia grind entirely lol)

All languages get translated... Mostly. Untranslatable words (think schadenfreude) will still be in their original language, but the magic will make sure the meaning gets understood by others. Sign languages and cultural hand gestures don't get automatically translated either. You can use real French if you want, just provide a translation/make it obvious what it's meant to mean in context for those of us who don't speak French as it would be understood IC.

Francis will probably recognize a thing or two about the locals -- especially the fae and beasts who are from French folklore (gargoyles, werewolves...). Otherwise, a lot of the game's mythos is a blend of Celtic and Nordic fairylore anyway, so there's a pretty good chance he'd be able to make some educated guesses if not have some actual recognition.