Comment by m0llusk

Good to know about, but the really great contribution that Mozilla has made to localization is in Project Fluent (https://projectfluent.org/) which enables sophisticated translation with asymmetric localization, progressive enhancement, and other features critical to high quality localization. If you are just swapping in some strings then the usual combination of gettext and Intl are probably fine, but high quality localization often requires more than that.