I have two active keyboard layouts - US english and russian. I have russian as my primary system language and the option "allow different input language for different documents" is active. But when I open any app keyboard automatically switches to russian, while I want it to be english by default. Is there any way to change the default keyboard layout without changing system language?
When I tried enabling the setting (Input source options: Allow a different one for each document), new windows seemed to just use the current input source.
Anyway, you could try running
sudo /System/Library/CoreServices/Setup\ Assistant.app/Contents/MacOS/Setup\ Assistant and selecting US as the default input method. (You have to go through the steps of creating a new account, but you can delete it later.) It modifies at least
If that doesn't work, try running
sudo languagesetup and selecting English, or
sudo defaults write /Library/Preferences/.GlobalPreferences.plist AppleLocale en_US and restarting.
Try the option Use the same one in all documents and with a keyboard shortcut you can easily switch between different input methods, for example,
Cmd + Space.