>>13233219He never said you can't write in English or any other language. In fact he once banned someone for saying "Don't type in Chinese" in his chat. But if you write in English there's less of a chance of him responding to you, he likes talking with his chat and wants to chat in the language he can speak properly. He can read English pretty well but remarked reading and speaking are different.
By the way, he added that Japanese only note after this Wachastars episode, with how decent he sounds it's no wonder he added it to warn idiots he can't actually speak English.
https://youtu.be/acAUnUEF7GI?t=1968 