If I am trying to ask a friend how his or her Japanese studying is going, is the following sentence correct, and which of the two indicated particles is more natural?

日本語　の/を　勉強するのは、どうですか。
Nihongo no/o benkyou suru no wa, dou desu ka?

If I were speaking very informally, could I say:

Nihongo wa, dou desu ka?

If so, could "wa" or "desu" or both be omitted? Could "desu" be replaced by "da"?

How would a Japanese speaker be most likely to ask this question to a friend who does not yet speak much Japanese?

Grammatically, either
日本語を勉強するのは、どうですか。
or
日本語の勉強をするのは、どうですか。
is OK.

For the remaining questions I need to know more detailed situation. What do you mean with the sentence? Are you talking to someone inviting to learn Japanese? Or are you asking someone who already is learning Japanese his/her progress? Or...? As you may know どうですか is a quite vague expression.

maz

Maz, konniti wa.

Thanks for the answers.

I had in mind talking to someone who has already started learning Japanese-- in English, the phrase "his/her/your Japanese study(ing)" would only be used in this circumstance. If I were inviting a friend to study Japanese, I would ask "How would you like to study Japanese (with me)?" rather than "How is your Japanese study going?"