Watashi wa baka janai baka wa anta?

Answer 1

I am not stupid. Are you stupid?

Note that the above is translated correctly despite poor Japanese grammar.

Watashi WA baka ja nai is translated as "I am not stupid."

Ja nai is the negative of desu or ja. Ja is a short form of desu usually used with qualifiers, but is sometimes used alone in colloquial speech.

Baka WA anta? makes no sense since "stupid" is the subject instead of the person, and denotes a new topic, which is incorrect. Ga should be used, as this second phrase is a continuation of the main topic, and relates to someone else. Ga usually denotes a sub-topic or redirected subject. This phrase translates to:

"Stupid is you?", assuming the conjugal is implied.

Instead, the more formal phrase: "Anta ga baka desu ka?" should be used, or "Anta no baka?" in female colloquial speech. Some males in the Tokyo region apparently use something like "Baka no da?" in colloquial, as well.

The reason for the parallelism in Japanese language is the same as why New Yorkers tend to have a very different dialect than the Southerners in America.