where his civilizations, including our own

But spoken languages are learned socially, via interactions with more knowledgeable individiuals, not though any formal education. No one ever teaches you to speak your native language; you just learn it through interactions with your family when very young. Also, keep in mind that dialects that aren't considered standard by the larger society are still internally consistent. The reason it's so hard for outsiders to mimic the speech of people who speak certain dialects is that there are rules and conventions for syntax and grammar that may not be known outside that group. You say something because that's what "sounds right" to you when you're fluent. (For example, my husband did not grow up in the Appalachain mountains like I did, and when he tries to mimic the way my family speaks, he can't get it right. He says things that don't follow the unwritten syntax rules, and it's very clear when I hear it that it isn't "correct".) Languages are like that, independent of education level or the extent to which they;re accepted as legitimate by the larger society.

Of course, whether or not JKR has put a good deal of thought into how House Elves speak is another question altogether. I always thought they sounded as if they spoke a sort of pidgin English.