We've been told that we should use "Bitcoin" as plural in the English version because that's the agreed standard, but I think this will have to be decided for each language separately - after all, every language has its own rules and behavior. E.g. I made the Polish version and I pluralized Bitcoins there ("Wyślij Bitcoiny") because I think that sounds better there. In Polish we generally add inflections to all foreign names because they sound really awkward otherwise when used in Polish sentences; e.g. "in Windows" would be normally said as "w Windowsie" (treating "Windows" as a singular noun, because if something ends with "s" in Polish it's not a plural), or more rarely: "w Windowsach" (pluralizing it first to "Windowsy").
So if you feel that sounds better in French, leave it like this. It's a new territory so whoever comes first makes the rules
Alternatively, you could look at French Bitcoin sites and check how they use the word there.
