It will certainly depend on the group and GM but it will likely be less of an issue than you believe. Your best approach would be to begin applying for games and work with any interested GMs to iron out any potential issues. I always advise patience when applying for games as there are far more players than GMs so finding a game can take some time. Your written English is very good so there is always the option of text only gaming to get up and running but I suspect your spoken English will also be more than sufficient to not be a detriment to a game... likely a million times better than my spoken French! Final thought, there are a multitude of nationalities and languages running games on Roll20, finding a French speaking one is likely easier than you'd think :) Best of luck