Breakdown of Vilket språk är du dålig på?
vara
to be
du
you (singular)
språket
the language
vilket
which (ett) / what a (ett-word, exclamatory)
Questions & Answers about Vilket språk är du dålig på?
Why is it vilket språk and not vilken språk?
Because språk (language) is an ett-word (ett språk). When asking "which", the question word must match the noun's gender. You use vilket for ett-words, vilken for en-words, and vilka for plurals.
Why do we use på here for "at"?
Prepositions rarely translate perfectly between languages. In Swedish, the set phrase for being "bad at" or "good at" a skill is always dålig på or bra på. It is highly recommended to memorize på together with these adjectives.
Is it grammatically correct to end the Swedish sentence with the preposition på?
Yes, absolutely! Just like in English ("Which language are you bad at?"), Swedish very naturally leaves the preposition at the end of a question when the question word is the object of that preposition.
Since språk doesn't change in the plural, how do we know this means "Which language" and not "Which languages"?
You can tell by looking at the question word. We used the singular vilket, so it must be asking about a singular language. If the speaker wanted to ask "Which languages", they would have used the plural question word vilka instead, making it Vilka språk är du dålig på?.