Steps to reproduce it:

1. Login as admin
2. Install the Catalan (ca) language pack
3. Go to Site administration > Language > Language settings
4. Set "Language autodetect" = Yes
5. Set "Default language" = Catalan
6. Save changes
7. Logout
8. Set your preferred browser language to Spanish
9. Clear your browser caches and cookies
10. Go to the Moodle login page

** You can replace Catalan/Spanish by any other languages you're more familiarised (except English).

Expected result:

The page is displayed in Catalan as this language was autodetected for you

Current result:

The page is displayed in Catalan only if Catalan is defined in the "Choose your preferred language for displaying pages" list (Firefox) or "Settings -> Languages -> Language" list (Chrome). 

For fixing this issue Andrew suggested we could do something to say "Use $CFG->lang when priority is < 0.9".