"Certains Français mangent des escargots."
Translation:Some French people eat snails.
Masculine Singular = Français. Masculine (or Masc+Fem) Plural = Français. Feminine Singular = Française. Feminine Plural = Françaises
Yes. Notice the plural "certains". Other context will also be a clue, i.e., it's not "un certain xxx" (singular) but as a plural determinant before a noun, the noun must be plural as well.