post https://{instance_zuid}.api.zesty.io/v1/env/langs
This call will replicate all content items marked with the new language. The default language will be copied into the the new content items. For example, if english is your default language, and you add German (DE), a new item for each content item that exists will be created with a unique ZUID, the lang "DE", but the starting content will be copied from the english version.
List of all accepted codes can be found here.