Auden's phrase "Still waters run deep" is translated, by Spanish poet Gil de Biedma as "la cabra tira al monte."

This is not even the case of finding a cultural equivalent. The two expressions are not even close to being equivalent in meaning. Yet, in a free version, this seems fine to me. Auden is quoting popular discourse, stringing together clichés.

At last the secret is out, as it always must come in the end,
The delicious story is ripe to tell to the intimate friend;
Over the tea-cups and in the square the tongue has its desire;
Still waters run deep, my dear, there's never smoke without fire.

Behind the corpse in the reservoir, behind the ghost on the links,
Behind the lady who dances and the man who madly drinks,
Under the look of fatigue, the attack of migraine and the sigh
There is always another story, there is more than meets the eye.

For the clear voice suddenly singing, high up in the convent wall,
The scent of elder bushes, the sporting prints in the hall
The croquet matches in summer, the handshake, the cough, the kiss,
There is always a wicked secret, a private reason for this.

En los cafés de la plaza / las lenguas la están corriendo
--Que la cabra tira al monte / y nunca hay humo sin fuego.

"La cabra tira al monte" means "the goat runs back to the mountain." In other words, things return to their natural state, you can take the boy out of the country but you can't take the country out of the boy. It's used to describe someone who returns to their wild ways after a period of seeming reform. [Literally, it is the observation that goats will tend to run up hill in search of better grazing.] The expression seems so prototypically Spanish that it allows Gil de Biedma to leave behind Auden's world of croquet matches, golf outings, and tea houses. There is a difference between café on the plaza and tea on the square. Gil de Biedma uses the "ballad" form, the "romance." This also makes the poem seem as though it were written originally in Spanish; it domesticates Auden's very British sounding poem.

We find Jaime Gil de Biedma's poem in a book by Jaime Gil de Biedma, not in a book of Auden's poetry translated into Spanish. Auden's poem might possibly be judged to be better than Gil de Biedma's, but not because a translation is necessarily inferior to the original.

I've always found it interesting that people do not know what proverbs mean. They could not always explain the meaning of a proverb in the absence of all context, even though they might be able to undestand it when they hear it in a particular situation. "Still waters run deep" seems simple enough. But does it mean "Watch out, though the situation seems calm there is danger lurking"?