In chapter 5 of The Hitchhikers Guide To The Galaxy, Ford Prefect shows Arthur the Guide:
Ford handed the book to Arthur.
"What is it?" asked Arthur.
"The Hitchhiker's Guide to the Galaxy. It's a sort of electronic book. It tells you everything you need to know about anything. That's its job."
Arthur turned it over nervously in his hands. "I like the cover," he said. "Don't Panic. It's the first helpful or intelligible thing anybody's said to me all day."
A screen, about three inches by four, lit up and characters began to flicker across the surface.
"You want to know about Vogons, so I enter that name so." His fingers tapped some more keys. "And there we are."
The words Vogon Constructor Fleets flared in green across the screen.
Ford pressed a large red button at the bottom of the screen and words began to undulate across it. At the same time, the book began to speak the entry as well in a still quiet measured voice. This is what the book said.
"Vogon Constructor Fleets. Here is what to do if you want to get a lift from a Vogon: forget it. They are one of the most unpleasant races ..."
Arthur blinked at it.
"What a strange book. How did we get a lift then?"
So the guide has "Don't Panic" in English on its cover, and it shows characters and speaks in a language Ford Prefect assumes Arthur understands as he turns it on before putting the Babelfish in Arthur's ear (that happens at the end of the chapter).
Therefore, is the guide in English?