No. The Title of “Sir” is a British Chivalric title as given to a Male (Women get Dame or Lady) by the ruling Monarch. Let’s ignore that it come from the old French word “Sieur” for now.
So he should stay Sir Roostly in any language as that is his official Title. Like a military rank it transcends the very basic “Mr, Miss, Mrs & Ms”.
I.e if Sir Roostly was working in a foreign country, possibly an Embassy in say Atlantis or Valhalla then his mail should always come with “Sir Roostly” written in English. They could follow with the rest in Greek or Norse if wanted but they would be politely reminded that the Sir should stay as given.
Is my view. Though I guess if Poseidon wrote it in the Greek equivalent they would need to tread that diplomatic faux pas carefully I would imagine. But even gods need to respect the Chivalric system