Can an anime be in English only?
There is absolutely no law that specifies that an anime can only be in English.

I will state that this is incredibly rare, given that most anime are actually made in Japan for a Japanese audience (hence why they're in Japanese), but given that the Afro Samurai anime was released in English-only, and it and its original manga source work was originally made in Japan, this only goes to demonstrate that there can be no law against this.