When the Israelites under Jephthah wanted to tell friend from foe, they famously asked those claiming to be on their side to say “shibboleth.” Those who couldn’t—who mangled it as “sibboleth” instead—weren’t Israelites (Judges 12:6).
In English there are a few such hallmark sounds that non-native speakers find famously difficult, but we don’t talk about them much. I’m not referring to sounds that cause problems for native speakers of particular foreign languages, the way Japanese speakers find it difficult to tell “r” from “l,” or Germans have a problem with the “w” sound. I mean sounds that tend to baffle all non-native English speakers, with few exceptions.
On reflection, I think two are the most obvious. One is the short “i,” which seems to exist in few European languages. (This actually separates Americans, Canadians, and speakers of standard BBC English from natives of Australia and New Zealand: Aussies proverbially say “feesh and cheeps”; Kiwis, “fush and chups.”) The other is the “th” sound, which sounds in many other languages like “z.” I’m not saying that no other languages possess these sounds. Greek obviously has a th, with its own letter to go with it, and if memory of a long-ago class in the rudiments serves, Russian has a short “i.” But in general, these are good shibboleths in standard English. Ask someone to say “this is a bit thick,” and you’ll separate the native speakers, and excellent foreign students of the language, from almost everyone else.
Am I right? Have I missed some obvious ones? And, to flip things: are there famous sounds in other languages that cause their speakers to laugh at almost all foreigners for failing to pronounce them correctly? (The nasal vowels in French sound likely, as does the “r” in German, which isn’t rolled as “rr” is in Spanish but produced near the back of the throat.) Aspiring sentries want to know.