https://github.com/openstreetmap/iD/blob/develop/CONTRIBUTING.md#translating gives instructions to:

Translations are managed using the Transifex platform. After signing up, you can go to iD's project page, select a language and click Translate to start translating

However, when clicking on Sign up on Transifex, after entering username/email/password, user is asked whether to start free trial (which will supposedly create new team and expire in 15 days?) or if they want to join existing organization (in which case one supposedly needs to select which organization). . .

Instructions should specify which of those should be chosen (and what extra info needs to be inputted there, if any)