<em:localized> tag, (like here: https://developer.mozilla.org/en-US/Add-ons/Install_Manifests#localized), but I don't see the equivalent for main description and main title of package.json. https://developer.mozilla.org/en-US/Add-ons/SDK/Tutorials/l10n#Using_Localized_Strings_in_Preferences just explain for translations of preferences. I also tried something like extensions.[addonID].description= "test", without any results.