In theory, you could write addons in whatever you want. But if your code is not supported by a JVM, your plugin would have to implement all the frameworks required to run it.
I wouldn't bother trying to do this - it's a vast effort which would force your Confluence to be run on an inferior platform
Of course, Connect addons that run on other platforms are a different story. See https://developer.atlassian.com/static/connect/docs/latest/index.html
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