When using a site definition like this: 

I expect the default locale to be "en" (for international purposes, where as en_GB will be UK specific). This works fine for the homepage when the locale is set to "en", for subpages however the DefaultI18nContentSupport doesn't default to the default locale but instead resolves to the fallback on subpages. Resulting in a locale of "en_GB" where no locale was provided in the url as opposed to "en". 

For home page (correct):

On subpages, actual:

On subpages, expected: 

So that on the default/global website in absence of any content the fallback is used, but we don't set the locale for the website since that should still be the default. 

I've resolved for our project, but I think this might actually be unintended behavior across the board. 

Fix: