Navigate to https://translatewiki.net/wiki/Translating:Wikidocumentaries
Create a user account, if needed
Start or join a translation
Translatewiki translations are updated twice a week, so you will have to wait for the changes to take effect.
If you add a completely new language, you need to have at least 25% messages translated before the language is exported to Wikidocumentaries. It also needs to be manually added to the codebase, so please reply to this ticket when you have worked on a new language and especially if your language is not added although there are more than 25% messages translated.