How to reset Google Keyboard language using ADB?
Just found out that the command won't work for Google Keyboard because the selected languages are saved in the key ENABLED_INPUT_METHODS of the Settings.Secure table.

You can reset the selected languages of Google Keyboard with this command:

adb shell settings put secure enabled_input_methods com.google.android.inputmethod.latin/com.android.inputmethod.latin.LatinIME