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What is the most classic/used keyboard shortcut for switching between input sources (languages)?
Except for cmd+space of course which is used by Spotlight.
If you want your shortcut don't interfere with spotlight you can use option + cmd + space or something else.
but as you said cmd + space is really good as myself use this and replace control + space for spotlight, and use option + space for QuickSilver.
I use option