We have multiple times seen and gotten feedback from customers that they've accidentally created content in the wrong language when for example English is supposed to be the master language.
This is usually due to them mainly working in one language and don't switch language branch just to create a page in English.
We have a few CMS instances where we use fallback language and a navigation with mixed language context. Once a piece of content has been created with a master language other than the intended master language (English it becomes confusing and a frustrating task for editors to re-create the page in the intended language. Just to have to translate it multiple times (some of our clients have over 3 languages active.
I propose that you add the feature to allow users to change the master language of a piece of content (page / block to another language, let the original master language become a translation.
This could be added as a feature to the existing Language addon that our clients editors are actively using.