There has never been an Emperor of Germany; you're presumably referring to the Hohenzollern German Emperors, who each bore the German-language title of Deutscher Kaiser. That note made, the Hohenzollern emperors are often described as such in English. Of course, as you note, 'Kaiser' is used; in a manner analogous to how 'Tsar' is usually used to describe Nicholas II of Russia and his forebears, or 'Shah' for Mohammad Reza of Iran and his predecessors (although neither of those two were actually Tsar of Russia or Shah of Iran, being both emperors). In a somewhat similar fashion, the Ottoman emperors were usually referred to as sultans in English (although their idiosyncratic (after the fall of the Mughals in India) imperial title was that of padishah).

So I suppose the answer is, then: i) it does get translated, and ii) there are other cases where a local title is used untranslated.