If you control the App Store, you can enforce a policy like this. And, the whole security model of Fuschia seems great for enforcing something like this: use object capabilities to grant access to everything and don’t provide a stable API for getting those capabilities in unsupported languages.

Unless you have the developers submit source code, it's not really possible to determine what language the source code was written in (if the developer doesn't want you to). For example, if there is a C API, I could write my code in Rust and with only a little effort, have it compile to something which could also have been compiled from something written in C. Even for languages with a more substantial runtime, you could "transpile".

The thing is, you don’t need to make it impossible, just difficult and officially say that it’s “unsupported” to make it a non-starter.