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Some do, and some don't. OSv (https://github.com/cloudius-systems/osv), for example, is also a unikernel in the sense that it can only run a single application at a time (it has no kernel-user boundary, nor support for multiple isolated processes). But is designed to run any Linux executables (using a dynamic linker which is part of the kernel) so it is not limited to a particular programming language.
The OSv project is indeed now moving much slower than it used to when we had an entire startup company devoted to it, but is not dead - it still has three committers from three companies - myself (Nadav Har'El), Waldek Kozaczuk and Timmons Player. It still works, and you are welcome to try it. OSv is not as minimal as includeOS, which of course depending on your viewpoint and use case, is either a pro or a con. OSv supports more types of hypervisors, and has support for a larger subset of the Linux ABI, can run pre-compiled, unmodified, Linux shared libraries, and supports multi-core VMs.