It depends a little bit of context, who you're talking to and what kind of mood you're in. - "Because": you could say this if you are being beligerante, or a child has asked you why fifty times. - "I don't want to talk about it": this is usually quite literal. - "It doesn't matter": I don't want to talk about it, or I'm worried it will sound petty, or boring. - "It's private": I don't want you to know. Probably many others, but you get the jist. It's quite contextual.

In English, they can use one of those things... - "Because": you could say this if you are being belligerent, or a child has asked you why fifty times. - "I don't want to talk about it": this is usually quite literal. - "It doesn't matter": I don't want to talk about it, or I'm worried it will sound petty, or boring. - "It's private": I don't want you to know. If you want to be a bit rude you could say 'Because I said it' -Потому что я так сказал/а In Serbian, we can also say the same Zašto - Почему - why and as an answer we could also say зато - поэтому

May be "it doesn't matter"? It's an interesting question, i'd like to see other answers