[whstl]

For those cases you have to bring the runtimes with you.

With JVM and CLR you can use JNI and COM to generate SOs/DLLs, and both of them can use any SOs/DLLs via FFI. There is also IKVM and Jni4Net that allowed Java code to run in .NET (or at least used to be, I last used it 15 years ago). Results may vary.

For other languages it can be a bit more involved: if there's no such thing as exposing as a library, you must embed the interpreter, which typically involves using C++.

It's not fun. This is why people end up using network requests.

If you can have a text-only interface, or even involve files, you can also just invoke the other app as a process.