I think that Tonberry itself is quite English dominated. If you ever run into someone, they're typically an English speaker, and anything in shout chat is generally in English (things like hunts - you'll notice a difference in how people look for parties when NM's spawn in Eureka due to it being cross-world). There's another server, Kujata which has a reasonable English playerbase, and there are also some English players scattered in small amounts across other servers like Aegis. I've found I don't really encounter JP players unless it's cross-world content going beyond Tonberry.

As for the acceptance of ENG speakers in JP groups, you'll notice PF's are typically labelled with either the auto-translate "Can you speak Japanese?" OR "jp only" at the end of it. It depends on the particular individual people you encounter in these scenarios, really, but for the most part, if you can just communicate with them, they won't mind. There's a few guides like the one Myon linked that'll help you out with communicating with them, and Google translate can actually be fairly decent for reading what they're saying (not so much for talking to them, I've tried it once and they said that they "don't understand Chinese" and asked if I was ok... oops).