I’ve just finished the translation system which now includes core translations files and a solid fallback mechanism. This way your modules can benefit from existing translations (in the core translations files).
All the translations used by the default modules are defined in the core translation files, making is easy to translate the core of the system. Of course, 3rd party modules should use their own translation files.
The translation files can be found in /translations . Pull request with new or improved translations are more than welcome!
Note: although comments are officially not supported in JSON files, MagicMirror allows it by stripping the comments before parsing the JSON file. Comments in translation files could help other translators.
If you built your own module, and you run into translation strings from which the core system could benefit (generic texts), feel free to send a PR with updated translation files.