Hotkey switching between spelling dictionaries
You could assign shortcuts to scripts like this:

tell application "System Events" to tell (process 1 where frontmost is true)
    set shown to title of windows contains "Spelling and Grammar"
    if not shown then keystroke ":" using command down
    tell (window 1 where title is "Spelling and Grammar")
        delay 0.1
        tell pop up button 1
            click
            click UI element "Deutsch" of menu 1
        end tell
        if not shown then click button 1
    end tell
end tell


It's probably not possible to use the setLanguage: method in NSSpellChecker in DefaultKeyBinding.dict.