Bug 892031: [Notification] the "update available" notification is not translated when the language is changed

 • use mozL10n.localize() to ensure all element carry their l10n-data attributes
 • fix l10n.js so that element properties can be pluralized, too
Merge pull request #11255 from fabi1cazenave/l10nUpdateNotification-bug892031

Bug 892031: [Notification] the "update available" notification is not translated when the language is changed, r=alive