We're planning to transition Firefox to a new localization API based on Project Fluent.
As a stepping stone for that, we'll want to land and enable the new API together with new registry for l10n resources (L10nRegistry) and new model for language packs.
The aim is to migrate away from StringBundle and DTDs, enable easy language switching and updating in the product and use Fluent Syntax for localization.
I carved out a new tracking bug specifically for the soft launch of L20n - bug 1402061.
I moved the dependencies for it there.