We have created a tool which you might find useful.
You can find it here https://github.com/madetoengage/episerver-master-language-switcher.
The tool will allow you to switch or convert the master language branch on a site.
It will also update any blocks used upon the page also.
Let me know if you need any help.
I have a multi-site setup and need to change the default language on one of the sites.
Is there a straight forward way to do this in Episerver, without effecting the other sites' configuration?