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Anyone wanting to write their own Webmin modules or contribute to the core distribution should first read the module writer's guide. For support, join the development mailing list, or email me directly.
If you want to work on core Webmin code, check out the page on using the Webmin Git repository, which is hosted on Github.
New and updated translations for Webmin modules are always welcome. You should read the documentation on creating translations, and then download the wbmtranslator Webmin module, which makes it easy to edit and submit new languages.