Currently store admin can change default language for online store.
The change will remove all existing translation of target language, then current content will be used as base theme of target language. The store owner needs to translate and modify all content via theme customization WEB UI.
Not only this, but also need to translate and update all content of all products.
I suggest a more efficient method, when a store admin change default language, the system just switch to exist translation. Then all translated contents can be immediately became default language. It can save time for store admin.