Oh, now you’ve gone and set off the nerd in me. I apologise in advance.
I’m definitely no expert and I don’t often read the same text in more than one language, but I think the boring answer is, as you say, that it varies a lot. The translator’s skill obviously, the way the text is written and probably also how close the languages are in structure. English and Swedish are fairly closely related so getting meaning across is usually very simple, but getting nuance right can be tricky.
Idiomatic stuff (“dog’s bollocks” or “break a leg” for example) can be really hard because you can’t translate them directly and there might not even be a corresponding idiom in the target language, but they usually lend a lot of colour and character.
Puns and wordplay can be a big problem, and something British authors especially seem to love. There’s a bit in the Swedish translation of Terry Pratchett’s Soul Music where the translator has a minor breakdown in the middle of the book and writes a long footnote where he starte with “Okay, I give up” and proceeds to explain in excruciating detail the various music puns Pratchett uses: The main character is named Imp Y Celyn, meaning “Bud of Holly” in Welsh for example, and they play “music with rocks” but of course the Swedish word for rock has no relation at all to rock music.
There are also examples of the translator handling it masterfully. I’ve been re-reading the Astérix comics in French (which I’m still terrible at) recently, where every name is a pun of some sort, and I’m happy to say the Swedish translators did a pretty amazing job of getting the jokes and the general vibe across.
Cockney rhyming slang is another thing I’ve come across. We just have no equivalent but of course a character using it in a story can say a lot about them. Poetic stuff in general tends to need adaptation rather than translation, but with something like this it’s not just about getting the meaning or the style across because it’s a localised historical phenomenon in itself. How on earth do you solve that?
I don’t know that much about Japanese but I imagine the different levels of formality and honorifics might cause some problems with tone when translating to more uniformly informal languages. Formal/informal speech can be tricky between European languages as well. English has one word for “you” while German and French uses “Du/Ihr/Sie” and “tu/vous” depending on context and the seniority of the person being spoken to (and wether you’re addressing a single or multiple people). So any deference or familiarity shown by formal/informal language in a French or German text gets lost in translation to English and has to be either ignored or recreated in some other way.
Along the same lines it gets even harder translating into Swedish. We don’t use honorifics or titles at all these days. We still have them in the language, but using them can come across as joking, very distancing or even rude, so you have to get across the deference and respect of calling people “mister, miss, mrs, doctor, professor, prime minister, sir, ma’am” etc. without sounding like your story takes place in the 1920s.
Then there’s accents. Another Pratchett-related example is Sergeant Shadwell in Good Omens. A major character trait is that he speaks in an unholy mix of British accents (with different connotations). The Swedish translator really tries and does about as well as possible, but it’s just not doable to achieve the same effect. I haven’t read any more recent Pratchett in Swedish but I imagine the Picts in the Tiffany Aching books were a nightmare for translators.
For an English example, I have read somewhere that some Greek plays translated into English gave the Spartans Scottish accents to get their rugged warlike character across compared to other Greeks. And that might be a solution, but it’s certainly not the same vibe to have Spartan general Lysander go “Ach, git tae feck, ye daft Athenian buggers!” (or whatever it is, I haven’t read them ).
And there’s just the character of the languages and how they’re perceived. English generally sounds “cooler” and more metropolitan to Swedish people, which is why it’s often used even in domestic marketing. It’s a little similar to how “engrish” is used in Asian marketing but with less nonsense. A text that sounds neutral to an English speaker in English probably sounds cooler when read in English by a Swede, and might sound a little dorky or silly to us when translated to Swedish. This is particularly true of modern stuff and even more so of sci-fi. It’s kind of hard to talk about hyperspace and laser guns in Swedish without it sounding very childish or inadvertently funny.
Lastly an example of something that’s just impossible to translate is John McClane’s “Yippee-ki-yay, motherfucker!” in Die Hard. It’s impossible to translate directly. You could keep “Yippee-ki-yay” to preserve the cowboy flavour but we don’t even have an insult crude enough to get across “motherfucker”. The Swedish subtitler settled for “Tjosan, din jävel” which is hilariously mild. I can’t translate it directly but it’s something like “Whoopsie, you bastard!”. It is funny in context, but definitely doesn’t have the same feel.