Steps to reproduce the issue:
1. Create a user with language Spanish
2. Create a site that only has English and German locales as available locales
3. Create some pages in that site.
4. Access to that site with the Spanish user.
5. If you switch the language to German a pop up message will appear showing alternative locales to the user (including the user default locale) but those locales may not be available in that site (user's locale is spanish but the site only supports english and german)