How do we know that Joan of Arc was French ?

She was born in Domremy, France, and was instructed by Saints Catherine and Michael to deliver France from the hands of the English invaders. She sought out the son of the deceased king of France, whom she regared as the heir to the French throne, and she led him to Reims, where she stood at his side while he was crowned Charles VII of France. Why would she have done any of this if she was not French?