What aspects of the English language will be difficult for future peoples to decipher?
Think of specific parts of our language and/or slang. What about our own language (English) will be difficult for future peoples to decipher in our language.
The following link is the article on the Mayans and the website as well. These links don't have answers. This Q is in the lesson of Mayans Code.
Lesson 6 Cracking the Mayan code article
Cracking the Code
Since your question is about the future, we can only speculate. I speculate that some of the following aspects of the English language will baffle peoples in the future, particularly if the English language ever "dies," or changes to the point that it is incomprehensible to a contemporary speaker of English.
a) Our use of conjunctions together with verbs: This puzzles me even now. Think of an expression like "Pack up" your belongings; is something going up? Or, "add up the numbers," "write down the information," "sort out the problem," "go down to the store," "pay up your bills," etc., etc. If these idioms ever die, someone in the future is going to have a hard time figuring them out (there's another one--what does "out" mean in that expression?).
b) Slang: As you mentioned, English slang is going to drive someone crazy some day.
c) Spelling: English spelling is far from phonetic; just consider words like rough, through, and castle. As long as English remains a vibrant, spoken language, its spelling is perpetuated--and understood--by force of convention. If it ever goes the way of Sanskrit and Hittite, its spelling may make it as mysterious as the Rosetta Stone.
Every language has its own quirks, foibles, and idiosyncrasies. Though most languages have common characteristics, it's the unusual or quirky elements which are the most difficult to learn. Undoubtedly there are many things which would be difficult for future civilizations to decipher when they one day study the English language, but idioms would no doubt be at or near the top of the list.
My experience with foreign exchange students, for example, is that even the best English language speakers and writers have difficulty with idioms because they aren't literal but are used in quite literal contexts. Idioms are expressions in which the meaning is not the same as the words. If I ask you to lend me a hand, I'm asking for you to help me, not let me borrow a body part. If I tell you to bring a white elephant gift to the party, I'm not referring to an actual animal. If I say you gave them a run for their money, there is no currency involved. These are figures of speech known as idioms.
What makes idioms so confusing to those who aren't native English speakers is knowing when words should be taken literally and when they shouldn't. Some day in the future, idioms will cause some definite consternation for translators and students of the language. For native speakers of English, though, idioms are a piece of cake.