"What is our common goal?"
Translation:Jaki jest nasz wspólny cel?
It would be if there was a noun phrase/personal pronoun before "jest". Like "On jest naszym wspólnym celem" (whatever it would mean) or "Zwycięstwo jest naszym wspólnym celem" (Winning is our common goal).
Also "Co jest naszym wspólnym celem?" would work as a translation and I added such an option. But as "jaki" kind of asks for description (an answer "Our common goal is very important" would be a valid answer), this does not take Instrumental.
Recently br0d4 explained the difference to me this way:
If the "What" stays on its own, without any noun phrase or a pronoun, like in ""what is this?", "what do you want?", "what did you receive?", "what do you miss" - then Polish should use "co" or "czego" (depending on the case needed by the verb.
If there is some noun phrase or pronoun mentioned ("what colour do you like?", "what is your question?", "what people are they?", "what is it like being a soldier?") - in Polish it should be a form of "jaki" (or "jak" in the last example).
This is still not a perfect explanation, but the best that we came up with, I guess. Here, "Jaki" is the default version, although "Co" could work as well, but I am unable to explain 'why' it also works. Still, that's a basis to understand the general rule even if there are exceptions to the rule.