[mikewarot]

I don't think you've described the problem adequately. A REST api implies a server... is this program supposed to implement the server locally, or connect to the cloud?

There are multiple cross platform environments, including Lazarus/Free Pascal if you want a GUI that can connect to the internet as a back end. (That's my decidedly minority favorite)

If you just need command line functionality, It's really hard to ignore Justine's work on actually portable executables.

Python is cross platform, but don't count on the scripts working without support/updates in 2 years.. they tend to change things and break compatibility.