So there is an obvious pro and an obvious con toward the concept that God's Last Message is actually written in English.
The obvious pro (leaning toward "yes, it's English") is waved away by the previous answer, that Marvin is reading out the letters. He's not just spelling it out; at 37 times the age of the universe, his visual sensors are too weak to read the message. Arthur and Fenchurch help him to a tourist telescope, so he can zoom right into the flaming letters. He then views each letter independently, reading out the letters. Some might say he's "translating" the message, but what exactly would he be translating? The Spanish word for cat is "gato." If I was reading the word out letter by letter, I'd be reading "g, a, t, o" not "c, a, t." To translate a message, you must have a whole unit of communication in your mind so you can convert it to other languages, as written and spoken languages have a grand variety of writing systems, grammars, vocabularies, etc. Marvin is giving immediate feedback of the individual letters he is seeing, and those letters are English letters, translating to English words.
The obvious con, is the statement that God can make anyone read whatever He wants to tell them. There's an un-sourced quote from Douglas Adams (http://asymptotia.com/2006/11/02/gods-final-message/) that states "When Marvin reads the message, it says, “We apologise for the inconvenience.” However, Fenchurch‘s reaction suggests that the message she saw was the Ultimate Question of Life, the Universe, and Everything (and that presumably, therefore, the Message is personalized to each viewer)."
The idea that the Babel Fish translates "visual language" is refuted in The Restaurant At The End Of The Universe, Chapter 22, as pointed out by Crow T. Robot on a question asking if the babel fish translates language (https://scifi.stackexchange.com/a/114285/64659). The Golgafrincham writing is described as "[looking] 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." Which is ironic, because (as we find out later) the Golgafrinchams are the direct ascendants of Human kind. If anything Eezeereed should be "more in tune" to any mental mapping provided by the Babelfish than anything else Arthur could have read.
(Opinion: One important detail to point out is that Douglas Adams enjoys coincidences, like "Belgium" being a horrible, horrible word in the rest of the universe. It's possible to surmise that, by coincidence (or through the use of the Improbability Drive, or the destruction of a planet designed by extremely intelligent beings in a plural zone), the Galaxy being hitchhiked through actually DOES use English as a galactic language, and the only reason nobody has noticed that that's one of the major languages on Earth is that Earth had already been marked as "Mostly Harmless" and nobody bothered to go check. That's not the only galactic "coincidence". For example, Earth being the only planet to base a game off of the Krikkit Wars, complete with the wooden portion of the Wikkit Gate key being the cricket ashes on Earth. Despite being a boring planet with little to no contact with the Galaxy at large, it (and especially Britain) show up a lot in the galaxy. Maybe it's a side effect of being in a plural zone...)
However, all that said, let's assume that God's Final Message is NOT actually written in English, and that Fenchurch saw a completely different message tailored to her. Why would we "hear" Marvin reading out "We apologize for the inconvenience?" Well, if we also assume that there's a "God Field" that converts the actual message into a religious experience for the person experiencing it... the message we hear would also apply directly to Arthur Dent. As the stories are written from at least nearby his personal point of view, we would hear God's Message as it applies to both Marvin and Arthur.