What about our own language will be difficult for future peoples to decipher in our language? Think of specific parts of our language and/or slang.
If we operated in a vacuum and made the presumption that future generations would not have the same method of communication in the language that we do, I think that our short hand method of using texting and computer language as a substitute for formal language might be challenging for future generations. For example, "LOL" as serving as substitute for "Laughing out loud" or "TTYL" as "Talk To You Later" might all be challenges for future peoples. The substitution and transformation of our language into texting or computer shorthand has happened quite openly and brazenly, and individuals need to accept its reality in the present tense. As a teacher, I wage a battle to keep it out of formal writing and presentation of thoughts in a written format, but I understand how challenging it is. I can only imagine how future peoples who approach it, presuming it is not fully embedded in their linguistic pattern of recognition.
Well, clearly any idioms or slang phrases that are based in today's context and society are going to create issues for future generations. You might want to approach this from another direction and read a novel like Oliver Twist by Dickens to see how many uses of slang are contained that are now obsolete. Language is incredibly textual, and although English is spoken in many different parts of the world, its particular form and shape is different in each place because of the culture and background. Try getting an American and a Brit together and see if they understand each other or not!