We know how reading works.

Suspension of disbelief is an agreement to change things - but you have to convey that they are changed, or else everyone applies the rules they already know. Healing magic is made-up and can work however you want. Getting stabbed is real. You don't get stabbed and immediately go "guess I'm fine," except through the application of something made-up.

Admittedly, reading is so commonplace now, we assume it's universal. Literacy is the rule we apply by default.

But D&D still specifies which languages your character can speak.