It's the same with <ch> and <sh> for me. It took way too long for me to realize that they are different sounds in English ((Standard) Swedish does not have any affricates), or that <z> is supposed to sound different from <s> (/z/ does not exist in Swedish).
If anyone pronounces <ch> as <sh>, <z> as <s> and <j> as <y> they're probably Swedish.
I usually use the analogy that a Swede pronouncing "cheap" as "sheep" is equivalent to English speakers pronouncing "skjuta" as "tjuta".
As a native English speaker, it took me a while to hear the difference between Swedish "tj" and "sj" (at least the way it's pronounced in Northern Swedish, like "schhh").
Yeah, the <ch> and <sh> being the same is usually my Swedish detector. It is surprisingly common amongst swedes speaking english, and not something that danes or norwegians do.
It does, but only in the onomatopoetic word "bzzz", as in the sound of a flying insect. And if you were to drop that sounding s, if you said that a fly went "bsss" around you, people would be weirded out, thinking the fly pissed on you or something. So pretty much every Swede can make the sound [z], and can hear the difference between [z] and [s], but only for this single word. It's so weird!
I moved to Sweden from Norway at a time when writing a check really was a thing :) It baffled me that swedes would pronounce 'check' with an initial 'k'-sound (as in "käk" or "käck"), which is not at all like the pronunciation in Norway (which is similar to the English pronunciation). What a strange country :-)
Similarly, Danish has kept the 'dj'-sound in English loan words like juice, where Swedish dropped the initial fricative. That tripped me up a couple of times while in Denmark...
Danish generally just adopts loanwords wholesale, never bothering to adapt the pronunciation. We have lots of words from English, French, and German that are spelled and pronounced basically like in those languages.
Now as for Swedish, what trips me up is that I've met so many Swedes that (otherwise) speak great English, but seem to pronounce J sounds in a distinctly Swedish way when speaking English. I've had fluent English-speaking Swedes in a university setting asking if I want to hear a "yoke" (joke) and or maybe telling me about their "yeans" (jeans). It's so weird. It's a bit like how many French people seem unable to pronounce an H when speaking English, but Swedes are damn good at English, unlike the average French person.
Yeah, the "blindness" to sounds that don't exist in your native language is funny, and usually the best way to figure out where a non-native speaker is from. If you can't even hear the difference between "sheep" and "cheap", or "joke" and "yoke", you have no way of pronouncing it right in the first place!
Likewise, I can identify English with a Danish accent pretty quickly, but I can't for the love of me speak it, because I'm not good enough at Danish. But a dead giveaway is that Danish realizes 'R' as [ʁ], and many Danes pronounce it the same way when speaking English. :-P And there's something with the vowels that's just weird that Danish people do... I can't put my finger on it.
(I have the opposite problem, I can't make that 'R' sound at all, which is why my Danish sounds pretty funny to native Danes.)
The Danish accent is definitely instantly recognisable, but I also can't put my finger on what it is. It's definitely something about the vowels. Danish is a very vowel-heavy language, so I'm sure some of that bleeds over into English.
I'm old enough that I have used a check a couple of times, but probably fewer than 10. I remember not being certain how to pronounce it "xeck" (käck) or "sheck" (sjeck) or even "tcheck", to improvise an English sort-of-phonetic spelling.
If anyone pronounces <ch> as <sh>, <z> as <s> and <j> as <y> they're probably Swedish.