It is incorrect. You do not say 誰は anywhere. There are two ways to correct your sentence.

誰が日本語がわかりますか。= Who (which one of you) knows Japanese?

誰か日本語がわかりますか。= Does anyone know Japanese (around here)?

It is correct. You are speaking to the person(s) that are right in front of you.

To explicate from a learner's perspective, you can never have a question word as a topic. Imagine the absurdity of this in English: "As for who, is Japanese understood?" That's what 誰は日本語がわかりますか translates to. Nonsensical.

Question words are never followed by は. It's が or some other non-は particle.

I have a question regarding the language exchange websites, which are the most popular among japanese english learners? I am thinking that I will learn more and faster if I start speaking completely in japanese (even if it's half broken).

Asking for help among bilingual people would be rude, and the language exchange ads in here seem to have very few replies , that's why I ask, what websites would you recommend?

what does this mean?
 

ネズミくんふいたｗｗ

That's going to need some context.
ww == lol because it looks like ハハハ.

Without context, I want to say it's "The mouse whistled." I can't conceive of a mouse playing a flute or something, which is also a choice, and while there are other meanings for ふいた, whistled (to me) seems most likely to generate laughter.

I think it's w because that's the first letter in 笑い.

Until you see someone say wﾊﾊﾊw. BUT! Wiki agrees with you. I stand corrected.

And as long as we're on the subject, my favorite is still wﾊﾊハハノ丶ノ丶ノ＼ノ＼

Japanese for "I have to go"
 

行かなければなりません

This sounds harsh because it seems like saying "I musn't not go"

Is there some other way to say it?

Context needed.

吹いた = played a horn instrument, bragged, etc.
拭いた = wiped

It does NOT sound harsh; in fact: it sounds very polite if anything.

Other possible phrases:
行かないと。
行かなきゃ。

It sounds more natural if one adds そろそろ in front of these two.

Just so you know, I'm a lawyer. Want me to sue you for copyright infringement?