When I look at historic and literary sources from the past 3000 years, from the epics of Homer to the present day, I cannot find any fundamental differences between the people that have lived then and those that live today, at least none that are greater than intercultural, class or gender differences among the present populace. We are all driven by the same basic needs and motivations, we have the same variety of character and temperament, the same faults and abilities.
I don't see why people 500 years in the future should be more different from us than people 3000 years in the past. Evolution is a slow process.
And as far as language goes, you ask as if you were unaware that not everywhere on the world today people speak the same language as you. How can an American writer write a novel about people from Mexico, Japan, or Native Americans, who all do not share his language?
Seriously, how is that a problem? SF that attempts to show futurity through future English has always felt ridiculous to me. Just write in your own language. People will understand that Japanese talk Japanese, even if their dialogue is written in English in an English language novel.
In a novel about Japan, how do you show readers that they are now in Japan?
- State it. Pleople don't expect everything to be different. Japanese are people. They eat. They wear clothes. So just say that we are now in Japan and give Japanese street names.
- Show the relevant differences. Japanese eat sushi. Explain what sushi is and how it is eaten. Don't talk about clothes, because they are basically the same, and you only need to mention kimonos when they are relevant to your story. (In a story set in the US you don't mention cowboy hats either, if they are not important, althought they signify the US in the eyes of many foreigners.)
The same goes for the future. Based on the assumption that much will probably be the same (people will still lie down on soft surfacs to sleep, sit down to eat, defecate in private, wear clothes in cold climate and develop or maintain a sense of physical decency etc.) you can leave out a lot. Just show the parts that are different and relevant to your story.
Even technically, a lot of inventions and changes have been predicted over the last decades, many of which never happend or never found their way into everyday culture. We still don't have an elevator to space. We still don't have electric cars (yes, we have, but so few that they are largely irrelevant in a novel about our time as compared to the 50s). We have the internet, but its just a way to connect. People will do that differently in 500 years, but they will still check messages, create profiles, play games etc. So really no need to show all the small differences. Just give some different sounding names (let people talk into their implants instead of a mobile phone or give them munti to eat instead of potatoes) and that's enough.