A babblefish is a small thin blue fish, not at all yellow or leech-like.
Remarkably similar to, although arguably somewhat less useful than, the babel fish referenced in The Hitchhiker’s Guide To The Galaxy.

It feeds on brainwave energy received from those around it, absorbing all unconscious mental frequencies from this energy to nourish itself with.

It then excretes (into the nearest mind) a telepathic matrix formed by combining the conscious thought frequencies with those from random thoughts such as the best way to slice a carrot or what it had for lunch last Tuesday.

The practical upshot of all this is that if you stick a babblefish in your ear you still can’t understand anything said to you in a language you didn’t previous understand.

Furthermore, you will also struggle to understand anything said to you in a language you do understand, both due to the telepathic ‘babble’ being excreted by the babblefish and, some might say more pertinently, due to having a small wet fish stuck in your ear.