Is it ok to play with English cards in FF TCG tournaments in France? 

Hi Atheris,

You can play with English cards this year. Last year they had put a constraint on the language (70% had to be in the language of the country) but this year there is no constraint ;)

Hello Lastdreamer, thank you for confirming this. 

Very glad to hear the English cards are now allowed. 

I need to purchase a couple of supporter pack from a nearby shop that just gets them in French. Is it for the most part worthy to appear at a competition with cards in various dialects?

Tutuapp 9Apps Showbox

No a problem as long as the competition itself doesn't require a different setup. Afaik, only the national competitive tournaments do not always allow full foreign decks, but that decision isn't always happening, so you need to check before going to the tournament by asking to the tourney host.