Yes, because they rely on the compiler (for a specific language) to build and deploy only what the application depends on.

The main examples I know of are Mirage (OCaml compiler) and IncludeOS (C++ compiler).

Though, I think there are experimental / research systems that try to understand a program composed of multiple languages that each compile to LLVM, or anything that compiles to JVM bytecode. So those might not be limited to a single language.

Here is a list of Unikernel projects, with their associated languages: http://unikernel.org/projects/

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.