I'd probably just use umlauts. Krum is a Germanic last name, which implies that his family were probably Volga Germans, or perhaps German Residents of Transylvania, rather than actually being Bulgarian by heritage.

 

That may be so, but should't Krum play for Transylvania then? It is mentioned in the books that the region has its own team. If not, his english would be similar to other eastern Slavs one.