を勉強してるの

are you studying …?

→ を
phrase Rare #40,540
are you studying …? The を marks the thing being studied. The の makes this a casual question.
JA: 日本語を勉強してるの?
EN: Are you studying Japanese?

どんな言語を勉強してるの?

What language are you studying?

何の教科を勉強してるの?

What subject are you studying?

いつからラテン語を勉強してるの?

How long have you been studying Latin?

今もフランス語を勉強してるの?

Are you still studying French?