If you are a translator new to localize WebExtension, you may want to read https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Internationalization and the locale-specific message reference.
The English strings in en_US are the reference for the other translations.
The messages.json files contains all localized strings used in the extension.
Note that you don't need to translate the description part.
That will never show up in the UI. It is only there to help the translating of the messages,
by e.g. making it more obvious in which context the message is shown.
The description.txt, developerComments.txt and privacyPolicy.txt are not directly used in the extension, and are optional.
They are used for the add-on description on https://addons.thunderbird.net/thunderbird/addon/dkim-verifier/.