Spelling and grammar, language selection per keyabord

You can always open a guest user account or second user account.

( a second user account would retain all settings between reboots.)

Set it up with one keyboard input language/ spelling /preference in particular.

Enable the quick user switching in the menu bar for easy back and forth.

>System Preferences>Users & groups>

macOS: Set up users, guests, and groups on ... - Apple Support

>System Preferences > Languages & Text > Text > Spelling

>System Preferences -> Language & Region -> Keyboard Preferences -> Text -> Spelling

How to use Auto-Correction and predictive text on ... - Apple Support

Change the language used at the login screen on ... - Apple Support

macOS: Change the language your Mac uses - Apple Support

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Markevic wrote:

I was thinking maybe there is option to included kind of fake spelling library for Lithuanian language, could be "aka empty", just to MAC think that he has spelling with no mistypes...

The problem is that in MacOS (unlike iOS) there is no connection between the keyboard and spellcheck. You can certainly add a real or fake Lithuanian spelling library, but the only way to use it is to select it in the Edit > Spelling and Grammar > Show Spelling and Grammar menu. Which is more work than just turning spellcheck off.

It’s not possible as far as I know. You have to turn spellcheck off separately via the edit menu for the app.

Perhaps some kind of Automator script could be built that switches keyboards and spellcheck settings at the same time.