Crowdin
From Joomla! Documentation
Crowdin is a cloud-based solution that streamlines localization management for your team. It's the perfect place to effectively manage all of your multilingual content.
The Joomla CMS official language packs and many other aspects of the project are managed using Crowdin to facilitate managing L10n activities.
Getting Started
Create an account
To start using Crowdin, create an account at https://crowdin.com
Joomla Translations
For more information on participating in the Joomla Translation Working Group join the Core Translation Team or find out more information on JDocs via the Translations Working Group
The also has a range of documentation articles that translators can contribute to.
Knowledge Base
Crowdin have a comprehensive Knowledge Base which new users would be well served to familiarize themselves with.