Likely, if there were simple, consistent ways to ask the other Klingon questions as commands, there wouldn’t be ANY Klingon question words. It’s quicker and simpler to ask the Klingon questions, and there is no simpler, quicker way to ask “Which”, except to command someone to identify the item in question.
You have no evidence that Klingons ask for identification by using imperative ngu' instead of having a question word for which because imperative ngu' is quicker and simpler. We are actually given a completely different explanation (see below). Sometimes the Klingon way is quicker and simpler, and sometimes the English way is. I do not find jar DamaSbogh yIngu' any quicker or simpler than What is your favorite month?
We do, however, have a bit of evidence that Klingon culture has
something to do with using ngu' where English uses which.
Okrand says, "When asking someone a question of this type, you are
really asking him or her to make a choice. So just be a Klingon
and order them to do so..." This may just be a quip by Okrand, but
we might also interpret it as an actual sociolinguistic
explanation. But it ONLY applies to ngu' translating which
because "being Klingon" here involves ordering someone to make the
choice. It doesn't apply to any other type of question.
-- SuStel http://trimboli.name