This userscript displays flag icons to Wikipedia languages list on left sidebar. See screenshots.

This is a open-source script hosted on GitHub:
https://github.com/DavideViolante/Wikipedia-Flag-Icons
- You can contribute to make it better
- You can give me suggestions or report me issues
- If you want you can donate :)

This usescript is also available as a Chrome extension here:
https://chrome.google.com/webstore/detail/wikipedia-flag-icons/hcpffdppeejlgoaagghjfnecaofbghgp

I hope you find it useful!