У дурака язык впереди ног бежит

Fools talk very much without thinking first See Язык болтает, а голова ничего не знает (Я)

Cf: An empty head, like a bell, has a long tongue (Am.). Foolish tongues talk by the dozen (Am.). A fool's bell is soon rung (Br.). Fools cannot hold their tongues (Am.). A fool's tongue runs before his wit (Br.). A loud mouth and a shallow brain go well together (Am.)