4 - The Buri language, while not particularly convoluted or difficult to understand the structure of (in fact, most words, phrases, and sentences translate rather smoothly), is extremely difficult to learn for the simple fact of - it is unlearnable by anyone a Buri has not allowed to "learn" it. 

That being said, the language is not necessarily "learned," one either knows it fluently, or they can barely speak a single word. All Buri know it from birth.