It seems like you want to use "true" to mean "true and useful", but I don't think that this agrees with what most people mean by "true".

You're right, of course. To a large extent my comment sprung from a dislike of the idea that mathematics possesses some special ontological status independent of its relevance to our world - your point that even those statements which are parochial can be translated into terms comprehensible in a language fitted to a different sort of universe pretty much refutes that concern of mine.