Stew You're doing it wrong, please see this reply from Toby concerning localization.
Be aware that the language extension system will be revamped soon, so if you don't want to work on your extension for almost nothing, you should wait until the new system will be released (released localization extensions will have to be revamped as well, because it seems it will be easier than just manually update the code). However, you can keep your actual translated files, they should be okay for the new system.
Concerning the other files to translate, they're related to extensions. You have to check all the extensions on the GitHub's Flarum Page. For example, concerning the "Mentions" extension, you should see the localization in the mentions/locale/en.yml file).
