Node-RED is totally JavaScript as it uses Node.js under the skin. There is a Python function node but there isn't a realistic way to write nodes in Python.
You can also use the exec node to call Python scripts.
You can certainly use Node-RED with Windows, I do all my development on it. You can call windows applications using the exec node or you can call out to PowerShell if you need more OS control.
A quick search showed a JavaScript extension for the Unreal Engine so maybe that would help?
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