Using computers, this should be possible. So we assume the government mandates every office to be paper free and no-one is allowed to keep important documents at home.
Books are replaced with e-readers, disks with cloud services, and so on.
The computer networks themselves are hardened through encryption. I assume this part works well - no reason why not. There may even be some measures built-in to keep them from working outside a given area, by measuring time delays and other stuff.
Even game makers have to update their software, and players who don't learn fast enough or make mistakes only non-natives would make get kicked out. Businesses and administrations even more so.
The changes to the language build on features typical to the own language which are also difficult for the people of other countries to pronounce, or to learn - sounds, sound combinations, grammar and so on. Every day, some small change gets propagated through the media, all laws and other often used texts are updated accordingly. Some of the changes are not propagated, but get introduced quietly - especially where a native speaker will catch on easily and a non-native not so easily.
Also, the use of references instead of talking plainly is encouraged. This way, only someone with access to the network and/or inside knowledge can understand a document.
New grammar rules are introduced which are basically encryption systems - so that words change their meanings depending on context of all kind, so that a sentence sounds like gibberish to anyone who grew up with the other languages, and so on.
Words which people in other languages may censor themselves are also used - like curse words, names of respected/historical people, names of gods, and so on.
Also words which have another meaning in other languages so their speakers trip up, and such.
People working in sensitive areas get concentrated into different places. They will be taught some specialised vocabulary and grammar, some of it even confusing to natives. Some of it easy to track so leaks can easily be found. Dynamic internet content makes sure people have their own accent, which is used as a finger print. New ideas are sometimes tried in this, sometimes in that area.
Sometimes, very large changes are pushed through, and people are sent to schools to learn them. Schools also exist for the people slow to pick up on the changes. The ones who stay too slow have to live in places where and work jobs in which no information of any value is released. The schools have a lot of "students" whose main duty is to find anyone who is not a native.
With other countries doing the same, people are employed to find ways to keep up with their developments, and the results are not only used for spying, but also to make it more difficult for the others to keep track.
Ideally, any differences in physiology, mentality, abilities and so on are also used to increase the distance to the other countries, and furthered through reproductive medicine and such - only the people who have the correct characteristics can become sperm or egg donors, get into jobs where people are more likely to multiply and so on.
Mutations which help pronounce unusual words, understand unusual grammar or such are actively searched and spread through the population in similar ways.
There is some pidgin based on older language to get trade agreements and such. The trade itself is done without talking at special sections of the border. Otherwise, the borders are closed.