The spelling is not a function of BTMail, it is your browser that controls that. You need to change the language in your web browser and if your browser has it "writing assistance languages".
It will be in the browser's settings somewhere which will be different in each browser so with out knowing your browser I can not be more specific.
Can I set UK English as the default language
Where?
Try right-clicking somewhere where you can enter text such as a search box or empty email. There's a good chance that spelling options will be there. This is Firefox & Chrome is similar.