Sorry, I try to cover everything I've tried before I post a question,
but I missed this one. I turned off all of the autoswitching of
keyboards & input methods; I have three different input methods
installed, but I use them only rarely, so I keep thes keyboard commands
turned off. Besides, the input language isn't switching when I do this.
IIRC, this issue popped up before I installed Farsi & Japanese input
methods.

Any other ideas?

Hi Joel,

As a guess, it may be coming not from Word but from the Windows Language toolbar 'hotkey' for switching keyboard languages.
Ctrl+Left Shift is one of the keysets you can activate for use in the Windows Control Panel for selecting a language.