Certain sounds serve as a shibboleth, a test, of a native speaker. The “r” and “th” can vary or even be missing depending on the language. Another interesting example is the sometimes non-sound “h”. This back of the mouth sound with no teeth is pronounced with significant differences, depending on the language.
In English, the “h” is clearly but softly enounced: I am happy that the bag wasn’t too heavy. The standard English “h” is not a throaty sound but nevertheless hearable. That said, many English dialects either eliminate the sounds, as in my friend ‘arry, or involve the throat, as in go to [ch]ell. So, the differences in the English “h” are mainly due to dialects.
By contrast, French has two formal different pronunciations of “h”: silent and enounced. In most French words, the h is completely silent: quelle heure est-il [quelur etil]. However, in a few words, mostly foreign, the h is aspirated and kept separate from the previous sound: la honte. The latter is pronounced [la hont], not [lont]. It is the dubious pleasure of every learner of French to try to remember which h’s are aspirated.
Russian is extremely xenophobic about its h, which looks like the English x. It sounds is a bit more throaty than the standard English h, but is not frequently used in words with Russian roots. One use is onomatopoeia, such as the Russian word хахатать [hahatatz], to laugh. In foreign words, it has been traditional to replace the foreign “h” with a Russian “g”. An example of this Gollywood, the capital of the American film industry.
Hebrew is truly challenging. There is the Hebrew ה [heh], which is the soft English h of hello. The ח [het] is released farther back in the throat, creating a sound like the Russian word above, often written in English as ch. The Yiddish/Hebrew/English word that exemplifies that is chutzpah. Finally, the Hebrew כ [chof] is pronounced like the ch in Loch Ness and may be written with a kh. Of course, these distinctions are more formal and in practice based on ethnic group. Oriental Jews (from the Arab countries) traditionally have pronounced them more distinctly, presumably because they lived in Arab speaking populations with similar sounds. In informal or quick speech, even many a native Israeli fudges the issue.
If speech is silver and silence is gold, the h, hidden, aspirated, or sent from the throat, is a real treasure for linguists.