わかる

How it works

が (subject) marks 日本語にほんご (Japanese) as the thing being understood — Japanese treats it as the subject, where English would call it the object of "understand". The verb 分かるわかる (understand) stays in its plain dictionary form, and there's no polite です (is) or ます (polite), so this is casual. With a rising tone, that plain form becomes a question all by itself — no question word needed.