"How do you say this in Japanese?"
と is the marker for what is/was said or what something is called.
I am (called) Tom. トムと言います。
He said yes. （かれは）はいと言いました。
What did he say? （かれは）何と言いましたか？
This particle is also indicate something that was thought.
I thought it was expensive. たかいとおもいました。
I think that I'll go to the store. みせに行こうとおもいます。
It can also be helpful to think of と as the quotation particle.
I am called "Tom".
He said "yes".
Or the thought bubble particle.
"It was expensive," I thought (to myself).
I think (to myself) "I'll go to the store."
「何と言いますか」('How do you say?') works as a set. Even when only 「何と？！」 is uttered, it is usually implied that the full expression is 「何と言っていますか？」 or something similar.
In addition, there is another meaning of なんと, which means 'how !'/ 'What a ___!' Placing なんと before 日本語, while it doesn't sound that natural, it sounds like you are saying 'How Japanese you say it!?'.
Absolutely (so long as your question is not whether or not DuoLingo will accept said change).
It is more usual for the topic to be at the head of the sentence, but not unnatural to place it as you have suggested.
For some reason, even though i received an email stating that this is now an acceptable translation: 日本語でこれを何と言いますか, it still is not accepted for me. What is incorrect about this?
my answer was wrong, however the 'correct answer' at the bottom of the exercise page does not match the "translation" at the top of this discussion page! The particles are used differently, for example: In the red box at the bottom of the exercise page, the 'correct' answer begins with これを however the 'translation' at the top of this Discussion page shows it commences with これは. No consistency even within a single exercise!