I tried recreating the error you receive but could not.
Did you try setting the required parameter for the language where it says it is required, and see if that helps?
Strangely, I did not see the option to open an issue/query on the linked github. But maybe an idea to contact to Mendix support?
However, before reaching Mx support, I’d suggest trying to reproduce the issue in a blank app.
EDIT – When you receive the error, you can simply open the double click the error and it will take you to the widget that has this problem. In my case the caption was indeed empty and all I needed to do was to provide one.
To fix this, you need to provide the caption in the language . However, I can see that somehow the widget remembers that the default language was something else and always creates <Header> for the new language. I don’t see that as huge problem, but if you want you can raise a new ticket at Mendix support.