What is your favorite language to pair with C in a mixed code base?

Nelua! This is the description from its own repo:

    Minimal, efficient, statically-typed and meta-programmable systems programming language heavily inspired by Lua, which compiles to C and native code.

In other words, if you like Lua syntax, you like extending your language's behavior via meta-programming like you do with LISP, but you also want to have C performance, then Nelua is your ultimate choice!

You can mix it with C without breaking a sweat; just have a look at https://nelua.io/overview/ and you will understand what I mean.

C# because you can have C++ and C# projects side-by-side in a VS solution and MSBuild will still build.