Welcome!

There are a fair amount of English speakers on the Japanese data centres, though we're still a minority.

Tonberry and Kujata servers, both on the Elemental data centre, are the "unofficially recommended" high English population worlds - or joining any world on Elemental will allow you to play with people from those worlds since dungeons are shared across the DC.

Also, a tip that will save you some queuing time if you do join a Japanese server: make sure you turn on all languages in the Duty Finder options, once you unlock that. Otherwise the game will try to only pair you with English speakers for dungeons, which on the one hand is good for communication, but not so good for wait times.