His first words had been in halting English, but from the way he had appeared to be listening in on the conversation between me and Karin I would venture that he at least understood our language. If he was a Servant who had been summoned legitimately, he would have been granted a bare minimum level of common knowledge about the modern era by the Grail, as well as the linguistic capabilities necessary to express himself to others naturally. However, now that I was trying to determine his true name, that was only serving to impede my search.
Fate/Requiem, Volume 1, Chapter 1