I present to you, the ugliest possible "solution":
- Enable support for assistive devices in System Preferences » Universal Access,
- and displaying the Input Sources menu in the menubar in System Preferences » Language and Text.
- Make sure the shortcut
Ctrl-F8 is defined for Move focus to status menus in System Preferences » Keyboard » Keyboard Shortcuts » Keyboard & Text Input.
- Make sure the Input Sources menu is the leftmost menu item that can be moved by dragging while holding
AppleScript Editor and write three scripts, each of them with the following code:
tell application "System Events"
key code 100 using control down # press Ctrl-F8
delay 0.5 # wait a bit, UI might be slow
key code 125 # press down to open the menu
keystroke "german" # name of your desired language, in my case tested using German
key code 36 # press enter
Save once for each language, switching out the name of the language. If you want to press different keys, or assign something different from
Ctrl-F8, substitute with the key codes here. You can also move the Input Sources menu from its leftmost position by inserting a few right arrow key presses.
Invoke scripts however you want, e.g. use your application launcher (Quicksilver, Launchbar etc.), or wrap them in Services using Automator, and assign them keyboard shortcuts in System Preferences » Keyboard » Keyboard Shortcuts » Services.