On a general note: the Empire is modeled on the Holy Roman Empire during the reign of Charles V, who has been quoted (apocryphal as it is) as saying: "I speak Spanish to God, Italian to women, French to men, and German to my horse."
If we look at the actual use of languages in the HRE of that time, then names would be mostly German, some French, Italian and Dutch. All of which could be Latinised. Slavic names would usually occur in their German or Latin form. The Empire equivalents would be Reikspiel (German), Breton (French), Tilean (Italian), Westerland Reikspiel (Dutch), Kislevian (Slav), and Classical (Latin). English, I am afraid, does not come into the picture at all.