I'm sorry I don't understand this argument. Let's say we're talking about something on a website, I say I want to be referred to by the pronoun "xir". "No,because I can't translate that into Swahili" seems like an odd response to me. It seems irrelevant to an average conversation I'm going to have. And even if you are planning to translate the conversation into another language you could just.... ask what would be reasonable in the other language?
Of course not, it wouldn't be a "no, you can't request this pronoun".
It would be "sure, but mind you that I have no clue how to talk about you in non-English languages". And in some languages it's more than just the pronouns - for example, in some languages verbs may be declined according to a grammatical gender (a concept that almost doesn't exist in modern English and not really inferable from just "xir").
