Gift horse

In English: Don't look a gift horse in the mouth. (See http://www.phrases.org.uk/meanings/117000.html)

In Spanish: (Argentina and Perú) A caballo regalado no se le miran los dientes.

In Polish: Darowanemu koniowi nie patrzy sie w zeby. = You don't check the teeth of a gift horse.

In Italian: A caval donato non si guarda in bocca. Literal Translation: One doesn’t look a gift horse in the mouth.

In German: Einem geschenkten Gaul schaut man nicht ins Maul. (same as in English)

In Russian: Дарёному коню в зубы не смотрят. - Similar to English.