What have languages got to do with it? Languages are just like standards the GP mentioned (Unicode, ASCII, etc) - you need to understand the language to read the message, just like you need to understand how ASCII encoding works. Using a [non made-up] language means that you are designing the message to be read by anyone who receives the content - just because they may need to do some work to understand it doesn't make it encryption.
And yes, I'm aware that the US used Navajo radio operators during WW2, and that it worked. However, it would have taken just one Japanese linguist who had studied native American languages to have recognised Navajo (not necessarily understand it, but use it as a crib). Which is why the US didn't use this method in Europe - because of German linguists and anthropologists efforts to understand the languages (the method was used during WW1). It's also the reason why the US used *actual* encryption devices.