I guess you need to decide which one you prefer more: The company of this friend of yours or the server. Maybe you could convince him to transfer back to Tonberry? To be honest, I am suprised. Why would your friend join a server with alot of Japanese players, if he does not understand the language so well?

I think the root of this issue, is in the reason why he left, because leaving only, because a server is full of English players, but said player is not able to understand the Japanese language so well.. is odd.

Personaly, I would stay at your current server. He left the server without talking to you, which to me means, he does not care if he playes with you or not. I know he is your friend, but ask yourself, how much is your friendship worth to him? Because his actions clearly shows, that he is oke without you.