I thought it would be cool if sites where users share algorithms had a special entry box that accepted code in a variety of languages, but only accepted constructs that could be translated succinctly into any of the supported languages.

Users would see sample code automatically converted and rendered in their chosen language.

Of course, this would not apply to code written for specific APIs, because the APIs may be unavailable or organized differently in different languages. Most Stack Overflow code samples are ineligible. Unless someone would take all this a step further, and allow code samples to make use of any APIs that could be accessed and translated among any of the supported languages.

Does anyone know about this being done (in a simple sense, not including API translation)? Would this be useful? – or are most modern languages similar enough that code written in any language can be understood easily by anyone (not including pointer syntax, as not everyone completely understands that)? – or is the sum state of APIs and features so fragmented between languages and competing libraries that the technical and communal feasibility of such a project is too steep?

Whether there's any merit in the idea, I'll leave up to you. To take it even one step further, I think it would be amazing if such a project could guide the convergence of languages into a common API and fundamental set of features, identical in behaviour regardless of the language code were written in. Imagine if you could write new libraries in every language simultaneously.

There wouldn't be need for a single language, that's not what I'm proposing. New languages would be developed only to provide a new way of expressing the same algorithms. Different people may find it easier or more rewarding to work in different styles. Some languages may be tuned for elegance, others for density and other particular aesthetics, like fonts.

You're absolutely free to continue the thought.