From Joomla! Documentation
What is Crowdin?
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.
Create an account
To start using Crowdin, create an account at https://crowdin.com.
Once you've created your account, search on Crowdin for Joomla, and you'll then have a list of projects that include the Joomla CMS translations, Joomla Community translations for aspects of the project like marketing materials and Joomla official website pages.
You'll also find a number of other Joomla components who are using Crowdin for their translations management, so you can assist with translating them too.
Joomla Documentation doesn't use Crowdin for translation as it's built into the wiki. See the Documentation Translation Quickstart guide for more information.
Joomla Translation Teams
The also has a range of documentation articles that translators can contribute to.
Crowdin have a comprehensive Knowledge Base which new users would be well served to familiarize themselves with.