I think you can just stores locales in l10nManager instead of computing there here and there.
So in L10nManager constructor, do:
[this.locales, ...] =
    [Object.keys(utils.getJSON(localesFile)), ...
Then you can drop localizeIni "locales" arguments and just use self.locales within this method.
(you would have to update localizeManifest and localize accordingly)