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."

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

"The Karabiner mentioned by @wayfarer did the trick for me.
Here's the rule to map Ctrl-Shift to Ctrl-Space:

{
""description"": ""Swithch input language by ctrl-shift"",
""manipulators"": [
{
""from"": {
""key_code"": ""left_shift"",
""modifiers"": {
""mandatory"": [
""left_control""
]
}
},
""to"": [
{
""key_code"": ""spacebar"",
""modifiers"": [
""left_control""
]
}
],
""type"": ""basic""
},
{
""from"": {
""key_code"": ""right_shift"",
""modifiers"": {
""mandatory"": [
""right_control""
]
}
},
""to"": [
{
""key_code"": ""spacebar"",
""modifiers"": [
""left_control""
]
}
],
""type"": ""basic""
}
]
}"

i changed mine to ⌘-\ which is almost alt-shift, just shifted one key to the right (works with my macbookpro international keyboard, the us keyboard has longer left shift and the \ key is above the ⏎ key)

"In some locales command-space and option-command-space change input sources by default. You can enable or change the keyboard shortcuts from System Preferences:

The shortcut recorders don't allow entering shortcuts like option-shift though. If you really want to use option-shift, you can map it to a key combination like option-command-space with KeyRemap4MacBook. This is triggered when you release shift:

<autogen>__KeyOverlaidModifier__ KeyCode::SHIFT_L, ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L | ModifierFlag::NONE, KeyCode::SHIFT_L, ModifierFlag::OPTION_L, KeyCode::SPACE, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L</autogen>
<autogen>__KeyOverlaidModifier__ KeyCode::SHIFT_R, ModifierFlag::OPTION_R | ModifierFlag::SHIFT_R | ModifierFlag::NONE, KeyCode::SHIFT_R, ModifierFlag::OPTION_R, KeyCode::SPACE, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L</autogen>

This is triggered when you press shift, but it requires pressing shift before option for shortcuts like option-shift-[:

<autogen>__KeyToKey__ KeyCode::SHIFT_L | ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L | ModifierFlag::NONE, KeyCode::SPACE, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L</autogen>
<autogen>__KeyToKey__ KeyCode::SHIFT_R | ModifierFlag::SHIFT_R | ModifierFlag::OPTION_R | ModifierFlag::NONE, KeyCode::SPACE, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L</autogen>

This requires pressing option and shift simultaneously:

<autogen>__SimultaneousKeyPresses__ KeyCode::OPTION_L, KeyCode::SHIFT_L, KeyCode::SPACE, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L</autogen>
<autogen>__SimultaneousKeyPresses__ KeyCode::OPTION_R, KeyCode::SHIFT_R, KeyCode::SPACE, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L</autogen>"