Spell checker

Select all the text in the document (Ctrl+A)

Go to Tools>Language Set Language
Click on the language you want e.g. English(US)

At the bottom of the dialog box there is an option called: "Detect
language automatically". If it is checked, then uncheck it.
Click on the "Default" button.
Click "OK".

Tools>Language Turn off the Automatically detect language feature
Tools>Options Edit Turn off Auto-Keyboard Switching.