As the title says, I am interested in why these words became so widely used even though "emperor" and "submarine" mean the exact same. It just seems strange to read about, e.g., "German u-boats" and "British submarines".

My first hunch as a layperson would be to suspect that WW1 propaganda played a role. Foreign and "exotic" words probably were a good way of demonising the enemy – and German submarine warfare was of course a big issue.

Yet, if this hunch is correct, I would like to ask the follow-up question of why this does not seem to have occurred in any other instances (that I know of at least). The Japanese emperor is not referred to as Tennō, for instance. Were other Japanese words borrowed for propaganda purposes in WW2?

Apart from that, do we know why, other than by chance, we sometimes use the native title of a monarch (Basileus, Khan, Khagan, Shah, Sultan, etc.), and sometimes do not (the Chinese and Japanese emperors)?