Changing the default language affects underlying language settings
'Drupal says "the base language is [X] and a translation of [language term X] is [Y]".'
That's not a message from Drupal, that's my representation of Drupal's thinking process.
As a basic example:
- Your base site default language is English, and you have a taxonomy with a term "Welcome".
- You translate that into German as "Wilkommen" and French as "Bonjour"
When you switch your base site into German, Drupal says: we have the base term in English of "Welcome", show the German translation of Welcome.
Now, using Domain Locale, you create site B and make German the default language.
According to Drupal now, as the base language is German, when you show the taxonomy term "Welcome", that's in German so it will display "Welcome" and not "Wilkommen".
What's the patch does, then, is keep the underlying site language as English, but adds in an additional setting to say "but what is the real Domain Locale default language" and then uses that in language negotiation.
So if you set the Domain Locale default language as German, it will use that when detecting what language should be served up when going to example.com, but will use English behind the scenes when calculating what the base language of t('Welcome') is.