Relative clauses in Japanese?
Basic relative clauses in Japanese are fairly easy, like りんごを食べた男の子はあそこにいる。What I'm confused about is a noun clause (I think that's what it's called). For example, "I don't know who to ask about this", or "I ask him what they were talking about". I haven't heard that many people using sentences like that, so I'm wondering if it's just a sentence that only exists in English? And if it does, is forming a noun clause in Japanese any different than a regular relative clause?

Question

Answer

Actually, I don't understand you question perfectly for my English. 
So, I'd like to write some of that type of sentences with its translation.

I don't know whom I should ask about this. = このことについて誰に聞くべきか、私は知りません。
*I didn't understand your example sentence so I changed a bit.

I ask him what they were talking about. = 私は彼に、彼らが何について話しているのかたずねました / 質問しました。

I want to know how many people can come my party. = 私は、パーティに何人の人が来れるのか知りたいです。

I don't know where my cousins live in now. = 私は、今いとこたちがどこに住んでいるのか知りません。

I know which cake is more delicious. = 私はどちらのケーキが美味しいか知っています。

I hope this was helpful and please ask more if you still have doubt.