Then just pick one of them. This may be simpler than introducing another language to the project.

What if you are interfacing between multiple projects? It’s not about introducing another language, it’s about integrating pre-existing systems.

Some web projects validate inputs in JavaScript, then in a possibly different backend language, then (unusually) in the database with CHECK statements. Even when it's all JavaScript the validations are different because the frontend and backend frameworks are different.

And some projects have multiple backend services written in multiple languages.

Then cue might be a good idea.