I do think it is a bad idea, especially if some of these languages are similar (I don't have much knowledge of Japanese and Korean, but I'd assume they could be close or linked at least ^^) Like learning Spanish and Portuguese at the same time, German and English, I think it could lead to confusions, if I had to study more than one language at a time, I'd make sure there's a clear (and far) enough distance between them. But of course, everybody has it's own way of learning languages and memorising vocabulary, so you could very well succeed in each and every language that you're studying simultaneously, but personally I doubt I could ^^ Good luck and keep up the good work ! :)

Pour le contexte de "c'est fini, j'ai jeté ca aux oubliettes", tu pourrais dire " It's water under the bridge now", c'est une expression qui veut dire sensiblement la même chose, similaire à "de l'eau a coulé sous les ponts".