What irritates you the most with your own language?
Mine is people who separate words when they write. I'm Norwegian, and we can string together words indefinetly to make a new word. The never ending word may not make any sense, but it is gramatically correct
Still, people write words the wrong way by separating them.
Examples:
"Ananas ringer" means "the pineapple is calling" when written the wrong way. The correct way is "ananasringer" and it means "pineapple rings" (from a tin).
"Prinsesse pult i vinkel" means "a princess fucked at an angle". The correct way to write it is "prinsessepult i vinkel", and it means "an angeled princess desk" (a desk for children, obviously)
"Koke bøker" means "to cook books". The correct way is "kokebøker" and means "cookbooks"
Fellow Norwegian here. Seems like you've encountered a classic "sær skrivingsfeil". (For non-norwegians: The type of mistake described in the main post is called "særskrivingfeil", "sær skrivingsfeil" means "odd/weird writing error" and is itself a mistake of the "særskrivingsfeil" type.)
Personally I would probably answer the sj/kj issue, but I saw that you've mentioned it in a comment, and after thinking a little about it there is a bigger issue I have: People don't love the langauge. What I mean is that Norwegian is a beautiful language with many amazing words, but because people don't love it there is a perception that the langauge is "limited" or "boring". I'd love to read books in Norwegian, but the fact is that most authours/translators I've come across aren't very good at Norwegian, and it makes the book worse to read. Part of this issue is with machine translation. I was talking to a family member about this, and he mentioned that he had noticed a trend in the Donald Duck comics (which are/were hugely popular in Norway) from when he was young, and the lead translator of the comics was a teacher of Norwegian who loved the language, and the newer ones, after machine translation has taken over, and the difference was night and day. However, just to not be entierly negative I'll give you an example of someone who did this well: the people who translated the Spook's series (Den Siste Lærling) did a stellar job in my estimation with giving the names of things good Norwegian names and generally translating it well.
English, on the other hand, I feel like has not suffered as much from this, because they have benefited greatly from prominent writers who loved the language. I'm talking particularly within the sphere of fantasy, as that is where I am most familiar, where people like Tolkien and Gary Gygax are both extremely prominent writers who loved English and would use all those words that would (I think) have fallen out of the language if they hadn't put them in the public eye. I also think that while others who aren't as invested in the language would go on and write later, they would borrow some of the style from these earlier writers, because that's what the genre "sounds like". I think Norway needs a movement like this. People who dig up obscure Norwegian words that they can use as lables for things, and by doing that thrusts those words into the minds of readers, who will look up the definitions of those words and have richer lexicons as a result.
I've hear the argument "Norwegian is a poor language" before, and people usually argue that the English language has many more words to choose from. When pressed, people like that are borderline illiterate and haven't written anything meaningful in years. And they're fucking horrible at english too