Yiddish: He can't count to two.

He's stupid or naive—or he presents himself as naive and innocent in order to cheat you.
SOURCE: Bernstein, Jüdische Sprichwörter und Redensarten.

ער קען קיין צוויי ניט ציילן.

Er ken keyn tsvey nit tseyln.

He can't count to two.