I really like Nim the most. It's Python spiritually, but with types. Types just really really necessary, and it does have FFI support so you can forward arguments either from a wrapper or use {.cdecl} directly. It's still not a barneskirenn (as we say in Norway), but definitely the most fun I've had using another language in my various sandboxes.
Nelua and Zig are close seconds. Both are just so easy to work with C-based FFI. Nelua is probably not safe to use, as it's still a work in progress. At least last I checked. Zig is very much ready to use. Maybe not your cup of tea, though.
Golang has a complex run-time and I don't recommend using it. It's definitely possible though as my sandbox does have a full MMU. Just expect integration to be long and ardous. And the ABI changes potentially every version.
Rust is one of the easier ones. I didn't find it fun to work with though. It has by far the best inline assembly, but too much fighting with the compiler. I know that people love Rust, and it is fully supported in my emulator.
C/C++ has the benefit of having the ability to have their underlying functions overridden by native helper system calls. Eg. replacing memcpy() with a system call that has native performance. It's too long a long topic to talk about here, but Nim and Nelua also falls under this umbrella along with other languages that can compile to C/C++.
Kotlin is definitely possible to use. A bit hard to understand how the native stuff actually works, but I did manage to run a hello world program in several sandboxes with some effort. I'm not 100% sure but I think I managed to convert a C API header directly to something kotlin understands using a one-liner in the terminal. I would say it scores high just on that. Again, just a bit hard to understand how to talk to Kotlin from an external FFI looking in.
JavaScript is of course possible with both jitless v8 and QuickJS. v8 requires writing some C++ scaffolding + host API functions, and QuickJS requires the same in C. Either works.
And I think that's all the languages I have tried. If you think there's a missing language here, then I would love to try it!