We accept GitHub pull requests. Please refer to https://threema.ch/open-source/contributions for more information on how to contribute.
To report bugs and request new features, please contact the Threema support team through threema.ch/support.
We manage our app translations through OneSky. If you're interested in improving translations, or if you would like to translate Threema to a new language, please sign up at https://threema.oneskyapp.com/collaboration/.