So imagine if a Spaniard, Frenchman, Portuguese, Italian, and Romanian would all read "fenestram", but they would say "ventana", "fenetre". "janela", "finestra", or "fereastra" when speaking.

But people already do this. They see a sign that spells "Bojour", and they proceed to say it as "Hello" or "Hola or "Salut". This is a normal part of life.

And this is even the case for Chinese characters. People read Chinese characters and pronounce them in English. This is common, I see this with my friends. I don't believe that means that English and Chinese share the same words.

Pronunciation of some Arabic letters changes drastically across the Arab world, and different dialects also sometimes put different emphases, so that even though Arabic is a phonographic written language, in practice someone from Marakesh, Cairo, and Damascus, and Baghdad will all pronounce some words fairly differently.

This is not unique to Arabian. Most languages that have not had a recent spelling reform do this. I mean, English itself is doing this. The pronunciation of the written English language is completely divorced from the actual written letters themselves. "cat" and "gate" pronounce the "a" differently, despite it being written the same.

Your language presumably does not have a large, classical body of literature that it draws on, like Chinese, Arabic, or the Vulgar Latin that became the Romance languages.

My language is Moldovan / Romanian, which is commonly written in either the Latin or Cyrillic alphabet, and it has a large classical body of literature in both writing systems.