Per my other questions, I had assumed until the end of "Thanks for All the Fish" that Arthur could read things from alien planets including the Hitchhiker's Guide because the Babel fish translated written text as well as speech. I cannot remember when, if ever, this is confirmed.
Is there any direct proof that the Babel fish allow one to read alien languages in written form, not just translate verbal communication?
Arthur reads a wide variety of articles that are vanishingly unlikely to have been written in English, leading to the conclusion that that the Babelfish does indeed translate text.
The "Advice Planet" brochure:
Ha! Brochure!" spat the old woman. She seemed to be waving her bat more or less at random now.
Arthur fished the crumpled-up brochure from his pocket. He wasn't quite certain why. He had already read it and she, he expected, wouldn't want to.
A sign on the Advice Planet
There was a sign by the entrance saying, "I just don't know any more. Try next door , but that's just a suggestion, not formal oracular advice."
The life story of the "Advice Oracle"
"This is, er, this your advice then, is it?" said Arthur, leafing through them uncertainly.
"No," said the old lady. "It's the story of my life. You see, the quality of any advice anybody has to offer has to be judged against the quality of life they actually lead. Now, as you look through this document you'll see that I've underlined all the major decisions I ever made to make them stand out. They're all indexed and cross-referenced. See? All I can suggest is that if you take decisions that are exactly opposite to the sort of decisions that I've taken, then maybe you won't finish up at the end of your life..." she paused, and filled her lungs for a good shout, "... in a smelly old cave like this!"
...
He was just about to embark on reading the photocopied life history of the oracle, when he was rather startled to hear a slight cough behind him.
Bartledanian books and novels
He preferred not to think about it. He preferred just to sit and read-or at least he would prefer it if there was anything worth reading. But nobody in Bartledanian stories ever wanted anything. Not even a glass of water. Certainly, they would fetch one if they were thirsty, but if there wasn't one available, they would think no more about it. He had just read an entire book in which the main character had, over the course of a week, done some work in his garden, played a great deal of netball, helped mend a road, fathered a child on his wife and then unexpectedly died of thirst just before the last chapter. In exasperation Arthur had combed his way back through the book and in the end had found a passing reference to some problem with the plumbing in Chapter 2. And that was it. So the guy dies. It just happens.
Various panels on the Heart of Gold
Arthur listened for a short while, but being unable to understand the vast majority of what Ford was saying he began to let his mind wander, trailing his fingers along the edge of an incomprehensible computer bank, he reached out and pressed an invitingly large red button on a nearby panel. The panel lit up with the words Please do not press this button again. He shook himself.
Signs on Hotblack Desiato's "Sunship"
"Well, just over here in fact," said Arthur, pointing at a dark control box in the rear of the cabin, "Just under the word 'emergency', above the word 'system' and beside the sign saying 'out of order'."
The welcome banner at the NowWhat spaceport
Arthur Dent had been in some hell-holes in his life, but he had never before seen a spaceport which had a sign saying, "Even travelling despondently is better than arriving here."
The handling of written language in Douglas Adams' work is deliberately vague. However, the Babel Fish is implicit.
Remember that towards the end Fit The FIrst of the original radio series, Ford Prefect introduces Arthur Dent to the book. He shows it as a handheld device with a screen and scrolls through the index to find the entry for Earth. Arthur can read it, implying the display is in English. Although this is just after Arthur gets the Babel Fish.
In Fit the Fifth, everyone is able to read the menus in Milliways, the Restaurant at the End of the Universe. This is irrespective of their origin.
In Fit the Ninth, we learn that outside the offices of the Sirius Cybernetics Corporation is a three-mile high illuminated sign bearing the company slogan "Share and Enjoy". We are told that, after an earthquake, the sign falls partly into the ground and now reads in the local language "Go stick your head in a pig". Clearly that is not the English alphabet. The explicit mention of language however, makes the joke. This leads to the robot's corporate song which famously starts "Share and Enjoy" and ends "Go stick your head in a pig".
All the characters are also able to read the control panels of the Heart of Gold and, in Fit the Sixth, the Hagunennon Battle Cruiser
The other answers aside, according to The Guide itself, the Babel Fish is explicitly for spoken communication only:
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.
There's a couple of caveats, though. The Guide isn't infallible, and there are some known entries that leave out important details. (For instance, the entry for Earth.) Second, the brainwave part itself isn't stuck to very well. Arthur understood the millions-of-years-old message broadcast from Magrathea, for instance.
In The Restaurant At The End of The Universe Chapter 22, Arthur can't read the labels on the Golgafrincham freezers:
He rubbed the frost clear and examined the engraved characters. To Arthur they looked like the footprints of a spider that had had one too many of whatever it is that spiders have on a night out, but Ford instantly recognized an early form of Galactic Eezeereed.
It says 'Golgafrincham Ark Fleet, Ship B, Hold Seven, Telephone Sanitizer Second Class' - and a serial number."
This proves that the Babel Fish does not translate written text, furthermore it gives us a possible reason he can read basically everything else the other answers mention: this scene takes place millions of years before any other events in the series, and even then the galaxy has had a standardised form of written communication, it may be that by the time the events of the rest of the series occur that Eezeereed has become so easy to read that Arthur can.
