知らない (shiranai) can be a pretty biting way to say you don’t care about something/someone’s situation, but it’s not vulgar. しらねぇ (shiranee) would be a more vulgar way to say the same thing, that’s probably closer. I don’t know if there’s a metaphorical idiomatic phrase like the other languages in the OP.
For vulgarity in Japanese in general, you should watch this: https://youtu.be/GcjbwkBQeO8
https://github.com/processone/fluux-messenger is an XMPP client by the ejabberd people that seems aimed at being a Discord alternative. I think it is intended to support voice and screen sharing eventually, though it looks like they want to focus on getting text chat worked out for the time being.