An English-speaker who died 50 years ago might have trouble making sense of a lot of current-day English, especially online communications using a lot of acronyms. What would he make of LOL, for example? “Little old lady”? “Lots of love”?
You will find a lot of slangy type terms on an abbreviated format like emails and tweets, opps, xes, but more formal writing has probably been more stable and slow moving since the printing press.
I have a fellow from Honduras who comes and does yard work for me. He brings his 8 year son along who has an El Salvadoran. The father and I speak Spanish because he has little English, and had majored in Spanish until Sputnik when I switched to a Science major. His father keeps saying to the son (in SPanish), “Now I want you to listen to this lady and learn to speak the “good” Spanish she speaks.” Sometimes I will say a common phrase I learned in Mexico, and he will say, “Now you are talking Mexican,” and we both laugh. I know when my Puerto Rican daughter-in-law is chattering with friends in Spanish, I miss words and meanings.