We call the monarchs of Germany, Russia and Ancient Egypt "Kaiser", "Tzar/Czar" and "Pharaoh" respectively, but the monarchs of France, Spain, China and Japan "king" or "emperor". Is there any sort of pattern or rule about when loan words are used or not for monarchs, or is it just random?
Possibly because of how recently we have been referring to them. The English have been 'dealing' with the kings of France and Spain (or Castille and Aragon) for a 1000 years and so the same word was used as for their native leaders.
German Kaisers and Russian Czars are a more modern introduction and had a novelty value. Similarly Pharaohs would only have come into common use with Victorian archaeology.