"When is your birthday?"
Translation:Khi nào là sinh nhật của bạn?
Yes, "... là khi nào" would be correct here.
"Khi nào" is an adverb of time. A rule of thumb is, if you remove an adverb, the rest should still be a complete sentence (maybe not meaningful, but still correct by syntax). "Ngày sinh của bạn" is just a noun phrase, it lacks verb to be a full sentence. So there should be "là" there.
This sentence is another mistranslation and confusing. It is "which day is your birthday" therefore probably the birthday is soon and a specific day of the week implied. Even if a general statement there ìs definitely no "when" here. Sure the question could be asking you to say the date. But this kind of interpreting should be left to on the fly verbal dialogue unless you cover the whole gamut of possible answers. When you are trying to translate script it's counterintuitive and counterproductive to omit literality