SUMMARY:
Cannot switch input method (language) via Bluetooth keyboard because CMD+Space is now allocated for calling out Spotlight, which has been used for the language switcher since 2010.
STEPS TO REPRODUCE:
1. Connect an external keyboard to iPad via Bluetooth (ex: Apple Wireless Keyboard).
2. Go to Settings > General > Keyboard > Keyboards > Add New Keyboard...
3. Add more languages. (ex: Korean)
4. Open Notes.app, type something, and hit CMD+Spcae.
ACTUAL RESULT:
Spotlight is called out.
EXPECTED RESULT:
Input Method should be switched (between English ↔︎ Korean, for example.)
REGRESSION:
CMD+Space for Spotlight is a new feature in iOS 9.0 beta 3. Before β3, the shortcut has been calling out the language switcher.
NOTES:
For OS X, CTRL+Space will be allocated for Spotlight if more than 1 input method are up in system. iOS should hire the same trick.