Square-enix doesn't want you to play with the Japanese.

[Hoshi]

I think incursion would be difficult with a language barrier. I would be uncomfortable doing that event with anyone who wasn't on a voice chat program, much less didn't understand my language. I don't think SE doesn't want different groups to play together though.

[Malithar]

I think incursion would be difficult with a language barrier.

How so? I can't say first hand if the warps are named the same or not in Japanese, but I've yet to do a run where they didn't understand which warp to take. Beyond that, it's just doing your party role, there's not really anything specific strategy wise that would require people to be of the same language, much less requiring voice chat.