French is "français" (feminine: française; plurals: français/ françaises). Here the word français is an adjective, so use a lower-case f. A Frenchman is 'un français', a Frenchwoman is 'une...
French toast: we don't know. The earliest mention of French toast is in a collection of Latin recipes dating back to the 4th or 5th century. Another mention is during the reign of Henry the 5th when...