Alt + Shift as language switch on OS X

By default in OS X, spinning through keyboard layouts is defined as ctrl+space. This mapping is turned off in favor of the trigger for Spotlight.

To modify this default mapping, you need to go to System Preferences->Hardware->Keyboard & Mouse->Keyboard Shortcuts. There you will find a list item for Input Menu. Select the checkbox and it will become enabled. From there, double click on the predefined shortcut on the right side of the list. Once the existing shortcut is selected and editable, you can type your preferred shortcut sequence.

Unfortunately, alt+shift by themselves are not allowed as a shortcut. You need to have a non-control key in the mix. The closest you can come is to use alt+z (using 'z', which is next to shift on the left side of the keyboard). You also have the option of setting the shortcut for the reverse order of the spin. alt+shift+z is a natural option for this.

Use Karabiner (originally KeyRemap4MacBook). Search for option shift pc. Ticking the first checkbox should let you remap to the PC behavior.

The Karabiner keyboard customizer for OS X can map alt-shift (or ctrl-shift) to switch languages