OpenSUSE's Open Build System would be great to ship independent packages, but those are again heavily tied to Unices, hence leaving other platforms behind.
As far as I can tell, one of the main justifications for most language package management systems is "we also run on Windows/OSX, which has no package management, so we'll invent our own". As a result, users of systems that do have sane package management get stuck with multiple package management systems, one for the distro and one for every language. Even then, I find it disappointing that nobody has built a cross-platform package management system for arbitrary languages to unify those efforts.
And you should add Linux to your Windows/OSX as being an issue, which Linux package management tool would you build packages for? All of them?
The end user package management provided by the OS should be for installing end user packages and the language tool for installing and publishing libraries and dev tools.
Precisely so.
When languages try to hook into existing OS-level systems, the people on the language end get frustrated by the way the people on the package-manager end don't hurry to rush out bleeding-edge versions of packages the second they hit Github. To the package-manager people, that's no big deal, their orientation towards stability and predictability makes them comfortable with waiting a little for the coffee to cool. But to the developers, who want to get their hands on the Latest and Greatest Right Now!, it feels like slogging though molasses.
So the developers eventually end up blowing their stacks and stomping off yelling "Yeah? Well fine, we'll build our own package manager then! With blackjack! And hookers!"
But just because I'm doing that doesn't necessarily mean I want, say, the latest unstable version of the window manager.
Given the diversity of OS in the IT landscape, which systems are those?
The only existing system that I might trust enough for this kind of thing Nix, and at least for Haskell it is a pretty solid alternative to their language-standard package manager. Unfortunately, its popularity is way lower then stuff like apt or yum which are pretty shitty for development compared to something like pip or cabal.