system
Given the following post, respond as if you're commenting on Reddit:
human
[Hitchhikers Guide]Does the Babel Fish work on written language or only spoken/auditory language?

It auto translates everything you hear but would there still be a language barrier with written language?
gpt
Exactly how the Babel Fish works depends on which slice of the Whole Sort of General Mishmash you reside in. Generally, at least in universes that operate according to sane physics, you will have one of the following two entries in your mk.1 Guide: 

> The Babel Fish is small, yellow, leech-like, and probably the oddest thing in the Universe. It feeds on brainwave energy absorbing all unconscious frequencies and then excreting, telepathically, a matrix formed from the conscious frequencies and nerve signals picked up from the speech centres of the brain ... The speech you hear decodes the brainwave matrix.

Those of you living in, shall we say, more *verbose* slices of the WSOGMM, may find this entry instead: 

>  The Babel fish ... is small, yellow and leech-like, and probably the oddest thing in the Universe. It feeds on brainwave energy not from its carrier but from those around it. It absorbs all unconscious mental frequencies from this brainwave energy to nourish itself with. It then excretes into the mind of its carrier a telepathic matrix formed by combining the conscious thought frequencies with nerve signals picked up from the speech centres of the brain which has supplied them. The practical upshot of all this is that if you stick a Babel fish in your ear you can instantly understand anything said to you in any form of language. The speech patterns you actually hear decode the brainwave matrix which has been fed into your mind by your Babel fish.

As you can see, although the details vary slightly, the central tenet remains the same: your brain requires speech, or at least information-rich sounds, to complete the translation process. We know that there are limits even to this ability, as described in the adventures of Ford Prefect and some ape descendant on a tatty little world that got demolished:

> ... He had grown so used to receiving an instantaneous and unconscious translation of everything he heard via the Babel Fish lodged in his ear that he had ceased to be aware of it, and he was only reminded of its presence now by the fact that it didn't seem to be working. Vague shadows of meaning had flickered at the back of his mind, but there was nothing he could get any firm grasp on. He guessed, correctly as it happens, that these people had as yet evolved no more than the barest rudiments of language, and that the Babel Fish was therefore powerless to help. 

It is possible, of course, that your particular universe has a Babel Fish that operates in a very different manner, but in that event it is profoundly improbable you are reading this explanation, and will instead have just read one that accurately summarizes how the Babel fish operates in *your* universe.