I’ve gone to a convention in Tokyo before. Pretty much all of it is in Japanese with the exception of some translated english words. Most Japanese do not understand verbal English but a decent amount understand most written English, especially around the Tokyo area.

Chances are everything will be in japanese. I would only recommend going to the Tokyo one if you plan on making use of your travel time and visit other locations while in Japan. (You can check out the new FFXIV Cafe and the Square Enix Cafe)