Trabalenguas  - tongue twisters

Probably the most well known in English is the Peter Piper tongue twister.

In Spanish (Castillian) the most well known (from asking people) is 'Tres tristes tigres' - Three sad tigers.

Tres tristes tigres tragaban trigo en tres tristes trastos sentados tras un trigal.
Sentados tras un trigal en tres tristes trastos tragaban trigo tres tristes tigres.

I'll let you search the translation!

In Catalan the most well known seems to be 'Setze jutges'

Setze jutges d’un jutjat mengen fetge d’un penjat; si el penjat es despengés es menjaria els setze fetges dels setze jutges que l’han jutjat.

Somewhat gory but the translation is:
Sixteen judges of a court eat the liver of a hanged man, and if the hanged man unhooks himself, he would eat the livers of the sixteen judges who judged him.

As with many tongue twisters it seems most people only know the first part so learn it all and amaze your Spanish/Catalan friends...