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

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".