I18N and L10N
We are hosting our localization effort using Weblate.If you would like to contribute to the localization process, feel free to ask us to add a language then you can contribute to translations.
The project : https://translate.tedomum.net/projects/mailu/admin/
If you prefer to work locally, Poedit is one of the best tools available out there. Feel free to commit on your own fork and pull request your translations.