Are there sounds in other languages that you find hard to pronounce that do not exist your mother tongue?
Have you found a learning technique that could help others pronounce the words correctly?
For example, rolling French "R"s, or in Spanish the "jota" can sometimes be challenging.
The Russian letter Х (pronounced khah) doesn't have an equivalent sound in English, so American students are usually told that they know the sound already, from the name of the composer J. S. Bach or from Loch Ness, famous for its monster. American students also tend to stumble when they encounter the Russian letter Щ (shchah), but are told that they say it effortlessly whenever they say "English church" or "fresh cheese."
It seems to me that Russians have problems with the sound in the words fur, girl, pearl, and world. Most Russians I've encountered seem to substitute the sound in or.
I have difficulty pronouncing some Spanish words properly. There's a difference between the sound of a single r and that of a double r, but I've been told that people don't know whether I've just said "but" (pero) or "dog" (perro) except from context.
I can do those double r's in Spanish but really have to concentrate on them. And when I do, it sounds rather exaggerated to me, but apparently not to the person I am speaking to.
English 'th' is quite a challenge for most who learn English as a foreign language. That includes Germans, French, and Poles like myself. Rolling 'r' or Spanish double 'rr' isn't a problem though.
Polish on the other hand has many such challenges for foreign (non-slavic) speakers. They find it nearly impossible to pronounce words like 'chrząszcz' (bug) or książka (book).
American students also tend to stumble when they encounter the Russian letter Щ (shchah), but are told that they say it effortlessly whenever they say "English church" or "fresh cheese."
You've inadvertently hit upon one of my pet peeves, NorthAmerican.
They are, indeed, often "told" this--it's the explanation used in countless textbooks over the years--but it strikes me as quite wonderfully bogus. When I say "fresh cheese" or "English church" there are, unmistakably, two distinct consonantal sounds /sh/ and /ch/, not the requisite elongated voiceless alveolo-palatal fricative of Щ (IPA =/ɕɕ/) . Even if you do manage to sort of slur it into one word (freshcheese, englishchurch) you're still not producing the modern Щ--the consonant cluster produced thereby (/ɕtɕ/) would be a historical pronunciation, not how щ is said in modern Russian.
Better advice is given to students of Mandarin, which has almost the same sound (represented in pinyin by 'x') but shorter (/ɕ/). Grin tightly and say "sheep" and you'll produce a close approximation of the needed sound. (Delete the 'p' at the end and you'll be saying the Russian word for cabbage soup.)
I'd also say there are other Russian sounds that you neglected to mention that are even more likely to cause English speakers grief: ы and certain softened consonants (especially ль and дь) come to mind. And the English sounds that Russians struggle with include 'th' (i.e. /θ/, /ð/), the 'a' in cat (/æ/), 'j' (/d͡ʒ/), soft 'h,' and any consonant cluster that includes a 'w'.
As for sounds that I, personally, find difficult, the bane of my existence is the Arabic ح (voiced pharyngeal approximant) which I can sometimes get right, but only by accident.
#3 I think another challenge of Polish is visual -- seeing those clusters of consonants and all the z's -- like the word for bug, with 9 letters and only one vowel.
Ref #3, didn't the Poles have vowells available?, on the other hand every Hawaiian(?) word seems to be nothing but vowells. I can't even spell it, the adjectival form of Hawaii
And yet another thing about Polish is that diacritical marks over consonants may change their sound from what you might expect when you see them; here is one of my favorite pianists playing Chopin: Witold Malcuzynski. The L, Z, and N of his surname carry those mysterious markings. The nearest I can come to pronouncing his name is something like mahw-tsoo-SHIN-skee, but I'm sure that at the very least I've got the N wrong.
#5
Yes, chrząszcz has both confusing spelling and difficult pronunciation (for foreign speakers).
#7
Your transliteration of Małcużyński is correct. Pronunciation shouldn't be difficult either: ł is pronounced like English w (like in win), c=ts or close to it, ż is pronounced like French j in jour, and ń like Spanish ñ.