Editing the core file was Plan B, if there was not a setting to use first.

I used your edit, changing ('DEFAULT_LANGUAGE_ADMIN', 1) to ('DEFAULT_LANGUAGE_ADMIN', 0) since English is o and Swedish is 1, but when I log in I'm still getting Swedish as the default in the admin. I also tested in FF, Chrome is my default) and got the same result.

I don't see DEFAULT_LANGUAGE_ADMIN defined any where.