Create translation teams on Transifex
Look into creating translation teams on Transifex, locking resources to specific teams, and so on.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information
GitLab is used only for code review, issue tracking and project management. Canonical locations for source code are still https://gitweb.torproject.org/ https://git.torproject.org/ and git-rw.torproject.org.
Look into creating translation teams on Transifex, locking resources to specific teams, and so on.