"Certains Français mangent des escargots."

Translation: Some French people eat snails.

[DerGoldmann]

Does escargot refer to freshwater snails or saltwater? Or perhaps both?

[A_Root]

They eat land snails, so neither. https://en.wikipedia.org/wiki/Helix_pomatia

[Shirlgirl007]

Is Français plural here?

[ongaku81]

Correct.

Masculine Singular = Français. Masculine (or Masc+Fem) Plural = Français.
Feminine Singular = Française. Feminine Plural = Françaises

[n6zs]

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.