FreeTube uses the Weblate project in order to crowdsource all it’s translations. However the activation of the languages have to be made manually.
Go over to the Weblate page of FreeTube and go to “View project languages”. There you can see all languages that are already being translated. You can simply click on any language that does not have 100% and start translating.
If the language you want to translate is not available, you can add a new language by logging in and inside the languages view, hit Tools->Start new translation.
If you want to correct an already translated phrase, you can do that directly via Weblate.
If your language has a sufficient enough amount of phrases translated (best case 100%), you can write us in the Matrix channel or create an issue here on Github. From time to time we will take a look at Weblate and add the languages that have enough phrases translated.