We have created a project on Crowdin for translating Slicer. Anybody can register a user at crowdin.com and start submitting translations for the Slicer project. Please give it a try and let us know how things work. Probably these translations will be available for Slicer Preview Release users in a few weeks. Update: We use Weblate for managing the translations - see details below.
For anyone interested in technical details of where the translations are stored and how language packs will be generated, see this discussion.