English Translation
What is the main purpose of your studying English?
Sounds unnatural. 何ですか shows this sentence is a conversation. We don’t say 主要な in a dialogue. 主要な is for more formal announcements or for writing.
Rephrasing from 主要な to 主な（おもな）makes the sentence better. But still a bit unnatural.
君 is also awkward. The connotation behind 君 in this particular sentence is 1) the speaker undermines you, or 2) the speaker is challenging your learning objective (or even scolding you for your laziness). Either way, this translation is somehow hostile.
The most natural expression would be:
いろいろ理由はあるだろうけど、なんで英語を勉強しているの？ (a question to your peer/friend)
英語を勉強する最大の理由は何ですか？ (a question to a stranger or a senior person)