Give the players some objectives that they can fail without PC deaths. For example, the party hears rumors that a merchant is willing to pay handsomely for an escort through a dangerous area that is well known for containing threats that should be exceptionally difficult for the players to handle at their current level. If the players attempt to take the challenge, despite obvious clues that they aren't strong enough, give them a very difficult encounter. It shouldn't be impossible, but you should expect them to lose. However, this gives you a logical way to defeat the party without having a TPK. Perhaps the players get ambushed by bandits led by a powerful warlock and some other foul creatures -- a few party members can get knocked unconscious, the merchant attempts to hold onto his treasure and loses his life in the process, and the bad guys get away with what is now revealed to be a powerful magical artifact.
Try to set up battles in terms of objectives, rather than win/loss by one side wiping out the other. Here are some brief ideas which I believe fall under creating lasting harm for a character without offending the player:
Villagers or other innocents are evacuating, PCs have to hold off the enemy for some number of rounds. For every round the PCs fall short, some set number of villagers are killed, and the attitudes of the survivors will be based on this.
Escort a VIP or item, as detailed above.
Players must fight through enemies to interrupt some dark ritual -- failure results in a strengthening of the enemy. Perhaps instead of focusing on their main objectives, players would then be forced to spend time/resources on defending (loss of the battle effectively puts the players on defense)
The players are faced with an almost impossible combat -- if they begin to lose and choose not to retreat, a rival adventuring party saves the day. However, now the townspeople view the rival party as the "true" heroes. PC reputation takes a hit, and whenever PCs get cocky about things, townspeople and rival heroes always seem to be around to point out the players would be paste on some dungeon floor if not for the bravery and courage of these other heroes.
Most of these scenarios end with either the players' enemies growing in strength as the result of an attack where the objective was not to kill the PCs, or the players' standing falling because they failed to be perfect. Again, these should be hard challenges the players have a chance of beating, not impossible situations.
Another idea revolves around player sacrifice, and again the above situations can be adapted to this. Give the players situations where failure means they have to choose how they fail. This can help remove the feeling of invincibility some players get, while at the same time letting the characters have a sense of virtue. Perhaps a situation where the characters, if they can't outright destroy their opponents, have to make some sort of choice -- do they let the bandit leader escape (which means he will take the treasure from this dungeon, and no doubt find the players again), or do they pursue him and fail to save a much-loved NPC from the environmental hazard he/she got caught in during the last encounter? Sure, the characters lost the fight, but they saved their buddy, and can feel good about that. Plus, they'll feel good when you inevitably bring these once-victorious enemy NPCs back so they can exact revenge :)
Latin (and to some extent Greek) used to be the lingua franca during the middle ages. Later on, French became the language of diplomacy and nobility. Everyone that mattered(1) speaks a local variation of said language which should still be understandable by another speaker. For example, Quebecois and French or American and English.
So, you could have such a language that all the PCs speak. They should be able to interact with everyone else. Now, make sure that each PC speaks the language from where they will go adventuring. If not, they will have to find a teacher and learn the language. This does not take that much time. You can learn everyday grammar and vocabulary in about three to six months of (hard) study. This is what I do for all my games.
Well, almost all my games. If the game is set in a bounded location, then only those languages that are around said location will be relevant. If I set a game in 14th century Venice, I do not need to worry about the PCs speaking Japanese. If I set a game in the Crusades, you better believe that everyone will learn Latin, French, and Arabic pretty damned quickly if they want to get anything done.
If you have boogly powers (aka magic or psionics or whatever), then learning languages could be done via it.
As a side note, Middle Earth started as a setting to play with the evolution of different languages yet most characters manage to communicate quiet well -- and were delayed at the gates of Moria because of a translation error!
Philology is just cool. And just because it is hard to implement in game setting should not be a barrier to trying it out provided that it enhances the enjoyment of the game.
(1) Why, yes sire, I do have blue blood... What about Peasants? They don't need to speak to outsiders, they need to work harder and pay taxes.
Best Answer
This is tricky territory. Assuming you're staying completely within the Agenda and Principles, you're not permitted to lie to the players about what's happening. You're also not permitted to have secret plans that aren't yet part of the actual play. This limits your ability to have unrevealed secrets that you know are true. The nature and fact of a PC's insanity, as you're describing it, breaks the MC's rules.
However, it's awesome and is totally the kind of thing that should come out of a cheesed-off Psychic Maelstrom, so the question is not whether, but how to do it.
What you need is an insanity where you can:
This pretty much eliminates any "what is going on???" experience you want the players to have around an insanity that they don't know their PC has.
So let's see how you can do this, by going through those items backwards.
Radical honesty
The solution is to not hide the insanity from the player. Instead, make them complicit with you in creating the behavioural effects of the insanity. To do this, you can't just say they're insane and expect the player to run their PC that way, because doing what you tell them is categorically not the player's job in Apocalypse World.
To get complicity, bribe them mechanically while being brutally honest. Describe the sensory experience, honestly say that it's clearly not real, and then offer them XP if they act as if it is and ask "What do you do?" They don't have to take the situation at face value then, and retain their player-agency. Also, if they're intrigued (and what are you doing, offering situations that don't intrigue the players), then they'll be very tempted by the chance to mark XP and may just dive right in.
And if they don't dive right in, that's fine. They still have to answer "What do you do?" and their actions will snowball like normal. Maybe they try to focus and see what's "really" going on (sounds like acting under fire, yeah?); maybe they flee the scene and go try to get their head together; maybe they try to ignore them and make a guess at what's really happening. Either way, you can build on that, and they retain control of their PC's choices — the bedrock of an Apocalypse World campaign.
Don't assume you know it's insanity
So the Brainer, Iris, has been messing with some weird Maelstrom mojo and you figure this has exposed them to insanity, but you're not straight-up saying "so you're going kinda insane, how do you feel about that", instead you're dropping these weird events and visions on them and seeing how they react.
You might be wrong. They might not be insane. The rules dictate that you cannot decide what these events actually are, because you have to play to find out. Maybe Iris goes to Fanny Malone and flops down into the ambulance's gurney, demanding for Fanny to figure out what's going on with her head. Fanny goes to work like a Savvyhead on Iris and woah, the scene(s) that result reveal that Princey's planted a chip in Iris' head.
So you play to find out what happens, because that will be way cooler that whatever pre-planned stuff you were cooking. Unlike soup, more cooks improve the game instead of ruining it, so you gotta let go your plans and assumptions and follow the game where it leads you, just like you do when you're a player with a PC.
So go with the insanity thing, yeah. Have it in your back pocket for inspiration of things to tell your PCs they see, hear, whatever. But as soon as play pulls you away from that being the answer to what's going on? Put your idea that it is an insanity in the crosshairs and pull that trigger, like a good Master of Ceremonies. Your players will thank you, because damn but does railroading suck the fun out of this game.
Tell them truly what they experience
Get real vivid and barfing-apocalypse-y with your descriptions. It sounds like you've got some weird, evocative imagery up your sleeve — use that. I know I said don't plan anything, but planning visuals is cool, that works, because visuals can always have their truth explained later, and in real play you always adjust, toss, and repurpose planned visuals anyway. You don't get stuck on them, the way you get stuck on secrets and plans, so they're not verboten and dangerous to MCs.
So barf that apocalyptica, and see where your players take it, and make no commitments about what it truly is until you're honestly revealing what it truly is.
Bribes! Really really: Bribes!
I'm repeating this, because I cannot recommend it strongly enough. Players love XP as a bribe. They will do the damnedest things in the heat of a scene with an XP riding on their choice. And if they don't take it, hooboy, have they ever made a strong statement about their character then!
Offering XP for choosing obviously-bad courses of action is pure gold. Try it a couple times if you're not convinced, and you'll see how nail-bitingly charged those moments become, and how big the snowballs can get.
And as a benefit to you, it takes all the responsibility off you: they are entirely in control of their destiny at that moment, working with good information, and any road to hell they are about to set their feet upon, they will never give you grief for because they chose it with eyes wide open. That makes MCing so much less of a weight. Share the load — bribe your players!