Formal:
* "Krow Kernow!" - This literally translates to "Welcome to Cornwall" and is a standard, formal greeting.
Informal:
* "Kudhys yn Kernow!" - This translates to "Welcome to Cornwall" and is a more informal, friendly greeting. It literally means "Be at home in Cornwall!"
Bonus:
* "Krow dhe Gernow!" - This is another option, which means "Welcome to Cornwall!" but uses the more formal "dhe" instead of "yn."
Ultimately, the best way to say it depends on your audience and the situation.