From Localization Lab Wiki
Proprietary Web-Based Tools
Transifex is the web-based translation platform used by the majority of Localization Lab supported projects at this time.
Open Source Web-Based Tools
Localization Lab supported projects using Weblate: F-Droid, SecureDrop, FreedomBox
Note that SecureDrop hosts their own instance of Weblate. You can register via the SecureDrop wiki page.