It's much harder for spoken language to be misunderstood among the population that a native grew up in,

Well, there's still register switching, which is an important part of the study of linguistics. A native English speaker might freely switch between the different ways to say the same meaning, depending on context and audience ("sorry" versus "my bad" versus "apologies," or "you're welcome" versus "don't mention it" versus "my pleasure").

There are perceived formalities, common membership in different groups, unspoken social relationships and positions that are reflected in speech.

These systems can be described with rules, and we can recognize that sometimes one register is inappropriate or poorly fit for a particular situation, and that some registers have different rules of grammar.