The tongue is a wild beast; once let loose it is difficult to chain.

[Es fiera la lengua, que si una vez se suelta, es muy dificultosa de poderse volver a encadenar.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 222 (1647) [tr. Jacobs (1892)]
    (Source)

(Source (Spanish)). Alternate translations:

The Tongue is a wild Beast, very hard to be chained again, when once it is let loose. [Flesher ed. (1685)]

The tongue is a beast, which once at large, is hard to recapture and to chain.
[tr. Fischer (1937)]

The tongue is a wild animal, and once it breaks loose, it is hard to return to its cage.
[tr. Maurer (1992)]