I promise to use it in English as soon as Germany becomes Deutschland and Japan becomes Nippon.
You're free to call Japan Nippon as long as you're fine with people raising eyebrows, sometimes not understanding what you mean, or deciding you're a pretentious twit.
The request that we use a character that doesn't even exist in the English alphabet (ü) is particularly ludicrous.
It would make even more sense, after all we lose letters because we write those sounds using other letters or letter combinations, however the "ü" in "Türkiye" doesn't have an analogue in the existing alphabet.
