I asked on Twitter and these are a few of the examples that people mentioned:
Build tools and compilers:
Linters and code formatters:
Managing command line tools:
Given that each language has its own specialties, a non-JS language may also be better suited for a given task. For example, OCaml/ReasonML, Haskell, and other functional languages support algebraic data types, which help with processing data structures such as abstract syntax trees that are used when parsing and/or compiling formal languages.
You can “dog-food” – apply a language tool to the code of the tool itself. As a consequence, working on the tool’s code base gives you a better idea of what you want from the tool.
If high performance is important, AssemblyScript may be an option: It is a strict variant of TypeScript that is compiled to WebAssembly.