Language arises out of social behavior, beginning with imitation and reinforcement in early childhood. Children who don’t learn language (by interacting with adults and older children) in the critical period of early childhood, suffer serious developmental problems. So language is fundamentally anti-solipsistic, even anti-individualistic: you only acquire it by being brought into a community of language-users.

And written language begins as an encoding for spoken (or signed) language: everyone learns to speak (or sign) before they learn to read, and learning to read starts with learning associations between written structures and spoken ones. (For English-speakers, that means phonics: the relationship between letters or groups of letters, and sounds.)

Meaning isn’t “assigned” solipsistically; rather it’s “acquired” from a community of use. A single user can’t decide for themselves that “dog” means squirrel. I suspect that if you look at the word “dog” and try to convince yourself that it refers to a bushy-tailed tree-climbing nut-munching rodent, you will be aware that you are doing something very silly, something deliberately contrary to your knowledge.