Interesting. The fact that there are no loops means that there might be something close to a well ordering of language difficulty.

Actually, I don't think you can draw such a conclusion. Notice that, generally speaking, the arrows move towards languages of greater age?

Or perhaps, 'primitive' or 'legacy' languages simply are more difficult?

It's definitely possible, but I don't think we can rule out the logical possibility that a language will typically only reference another language that came before it.

It makes sense, too- Considering a given language, another language that is much newer wouldn't have existed when the phrase was created, and a language of the same age or only a little newer, was probably not widespread enough to be referenced when the phrase was coined.

The notion of two existing languages as differing in age is problematic. They are ever changing. How do you compare the age of contemporary french to that of contemporary chinese? If you look at the list you see that it is always (except for the chinese) different language families meeting.

Japanese is missing. Does anyone know their equivalent?

Allegedly, "The Japanese use a string of nonsense syllables which imitate the sounds of unknown languages, especially Chinese"

http://www.omniglot.com/language/idioms/incomprehensible.php

^ Worth noting that 'chinpunkanpun' is the word; the rest of the phrase is accurate Japanese.

I thought about that for a whole when that post first came up, but came to the conclusion that there really isn't one. English would be the closest example of what people say they can't speak, but there's no "it's xx to me" that I'm aware of...

Except of course for the phrase "it's your mom to me"