It does not sound very natural in French. It is better to say "Ta lettre concerne quoi ?" or "Que concerne ta lettre ?'
I think 'relative to' has the same meaning in English as 'relating to' and 'related to' so should be acceptable.
'Related to' or 'relating to' in this context means what is the letter about - 'relative to' would mean what is the letter compared to - which would not make much sense in this context
The displayed translation "Your letter is related to what?" is correct, but awkward in English. Thankfully, Duo now accepts "What is your letter about?"
I put-To what is your letter relating >--and was marked wrong.I don't really know why
I put that and was rejected, even though it means exactly the same thing as 'What does your letter relate to?' but without the dangling preposition, so stylistically preferable.
The French sentence means "what is your letter about"
Your suggestion would be equivalent to "what about is your letter" - that doesn't really make sense in English does it?