Que tu mano izquierda...
 

Que tu mano izquierda no sepa lo que hace la derecha... How would you translate this into English?
Thanks a lot for your help!

The left hand doesn't know what the right hand is doing.
The right hand doesn't know what the left hand is doing.

Both are used.

'Que tu mano ...'
Formal register: Let not your left hand know what your right hand is doing.
Normal register: So your left hand doesn't know what the right (hand) is doing.

Matthew 6:3
... let not thy left hand know what thy right hand doeth ...

¡Qué bíblicos estamos ultimamente!

Another possible translation:

Que tu mano izquierda no sepa lo que hace la derecha.
Don't let your left hand know what your right hand is doing.

Thanks a lot for your answers. I thought maybe there would be a non-literal translation, a saying meaning the same thing, but, since the quote comes from the Gospel (I didn't know it, I read it on a very anti-clerical text) I guess these are the only possibilities.

The complete quote is: Cuando des limosna, no permitas que tu mano derecha sepa lo que está haciendo la izquierda, meaning that you should be charitable but not say it aloud.

But, if you take the first phrase out, the sentence turns into a complete different meaning. I think it's got something to do with hypocrisy.

Pero chico, tu no ibas a catequesis cuando eras pequeño?

I don't read hypocrisy into the saying.
We use it to mean exactly what you wrote, that you should do something good and not toot your own horn.
Sometimes we use the phrase disparagingly, as when there is a communication break-down:
Management practices aren't the same on each floor; the left hand doesn't know what the right hand is doing.

The phrase is the same in Spanish. We use it to mean hypocrisy, as much as for example: A Dios rogando y con el mazo dando.
In Spain, there is a strong anti-clerical tradition which dates back mainly to 19th century.
Actually, I extracted the phrase from a text of the kind I'm saying. And the attempt of the writer, Clarín, as quite often, was to give notice of the hypocrisy of the Church.

We have a few sayings that denounce hypocrisy:
Walk the talk.
Practice what you preach.
Actions speak louder than words.
Do as I say, not as I do.

Cuando niña, yo iba a una escuela Católica y recuerdo que una de las monjas explicó que ese pasaje quería decir:

La mayoría de los seres humanos somos diestros asi que obviamente uno daría limosna usando la mano derecha mientras la mano izquierda está robando ya que la acción de la mano izquierda no es tan notable. Es allí donde entra la hipocresía. Das con una pero robas con la otra.

Elaina:cool: