How can I write a set of functions that can be invoked from (almost) any programming language?

Question

I'd like to find a way to write an API that can be accessed from any other programming language via language bindings (or some other framework). Is it possible to do this? If so, which programming language would be the most suitable for writing a "cross-language" API? My goal is to create a single set of functions that I can access from any programming language that I'm working with, so that I won't need to manually re-write the entire API in each language.