Test:

Install a simple multilanguage site with a new Joomla installation.
Add a menu item tagged to ALL languages in a menu containing already a Home page set to a Content Language.

Or edit this menu item and set it to "Default Page" and save, or click on its Home icon on the menu list.

Expected result:
No change in this menu. An error message displays :
"A menu should contain only one Default home."

The existing menu item set to Home and to ALL languages in the other menu remains untouched.

Result:
No change in this menu. An error message displays :
"A menu should contain only one Default home."

BUT the existing menu item in the other menu set to Home and to ALL languages loses its Default Page status.

Consequences:
It does not break the multilanguage site, but will create issues if a multilanguage site is reset to monolanguage as Joomla MUST have a default Home page menu item at all time.

My findings:
The menu model setHome() method directly calls the JTableMenu store() method. (The bind() method isn't used in this case)
This method does not check the existence of another menu item already set to Home in another menu, therefore, it first stores and then only look if there are multiple menu items set to Home in the same menu.