Install build 645-156 on a Spanish OS and the default language in the selection drop down is English. It should be Spanish.
Still an issue in build 645, 693
I tried to duplicate this and could not. I to region tab of settings. Set region to Europe->Spain and added spanish as the principal language. Started the app and the language was Spanish
Tested in build 717 and passed.
Verified, Below are the build details OS: Windows 8 Installer : 160 Build: 774WinOnly