Offering a number of languages in an add-on is very important and largely increases the satisfaction and the number of users.
For the past 10 years, Video DownloadHelper has used Babelzilla to maintain more or less 30 locales. But Babelzilla is no longer maintained and anyway, they did not take the jump to WebExtensions.
Video DownloadHelper 7.0.0 released yesterday introduces a feature that could be generalized to almost any add-on around.
Basically, it’s a layer between the i18n API and the add-on that may overwrite the translated strings. It comes with its user interface to allow any user to modify any locale string. It has an export function for the modified strings, so a motivated user can make the changes, like adding a new language or modifying an existing translation and send the file to the add-on developer to be integrated in the next update.
I am sure having an equivalent feature available by default in Firefox would greatly benefit many users and developers around.