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