This is a question I've been trying to figure out for a while now after noticing in my History lessons Charles V being referred to as Emperor but the Russian leaders being referred to as Tsar and not Emperor. Everyone I've asked has no idea why we do this and I find it very odd how we refer to Wilhelm II as the Kaiser but then call Charles V the Emperor despite the fact the Holy Roman Emperors would have been called Kaiser. Does anyone have any idea why we do this?
IMHO saying that the Holy Roman Emperors would have been called Kaiser is somewhat dubious.
During most of the middle ages the imperial title was "dei gratia Imperator Romanorum et semper Augustus" - "by the grace of God Emperor of the Romans and always Emperor". And when the title was given in German it would be given as "Romischer Kaiser", but that was merely a translation of the Latin title into the local language, the vulgar tongue.
The difference between Wilhelm II and Charles V is that Wilhelm's official title was "von Gottes Gnaden Deutscher Kaiser", while Charles V's official title was "dei gratia Imperator Romanorum et semper Augustus". And Charles V was the political successor of Augustus, of Trajen, of Constantine, of Justinian, of Heraclius, of Constantine V, of Charlemagne, of Otto the Great, of Frederick II, and so on.
So, IMHO, in the case of some non Roman ruler who calls himself "emperor" it is reasonable to call him an empereur, emperador, imperador, tsar, or kaiser, but a Roman, "Byzantine", or Holy Roman ruler should be distinguished from such comparative riffraff wannabe emperors by being called the Emperor or even THE EMPEROR.