I'm not surprised. I have similar trouble with some Chinese consonants. Chinese has 21 syllable-starting consonants. Out of those I can't distinguish (by listening) four pairs: q/ch, x/sh, j/zh, and c/t. And I still hear B/D/G as English voiced B/D/G, even though those three Chinese consonants are unvoiced. Re-training hearing is hard.
Tones in Chinese are relatively recent. A few thousand years ago Chinese languages had no tones.
Also don’t forget that it’s not just the sounds, but the combination of sounds that may cause grief. I am told that Japanese and Italian use vowel + consonant or consonant + vowel combinations, in other words two consonants with no intervening vowel are not allowed. Thus German is a nightmare for the Japanese, and poses issues even for English speakers in words such as stress and spreche.
"Almost identical" seems ridiculous to me, a native English speaker. Different dialects may pronounce vowels differently, but every dialect of English clearly distinguishes all of the different vowel phonemes in English, and English uses these differences to distinguish words.
Two sounds may sound almost the same to a non native speaker of the language, but to the native speaker they will sound quite distinct. To understand why we have to understand how the brain processes sounds. When we are young, we create a representation for each sound in the language we hear, and then we learn how to control the mouth and throat to reproduce those sounds ie we create muscle memory. When we hear novel sounds from a foreign language, we automatically try to map them to the existing sounds in our internal map of consonants and vowels. Many speakers of a foreign language never master the new sounds, and continue to substitute their own.
I am a beginner at German, and I find it hard to hear what native speakers are saying when presented with simple phrases. For example,
durch and
dunkel are confusing. When a friend who speaks fluent German says
durch it leaves me baffled. What makes matters worse is that there are many accents, thus
nedrig has at least two forms where the final consonant varies. I have had my prononciation of numerous simple phrases checked by native speakers, and apparently it is quite good, but it is very hard work, and some sounds such as the soft ch drive me spare.
However, the adult brain can learn to hear and articulate new sounds in a foreign language. I learnt the Welsh ll and ch sounds and the French guttural r, and the two oo sounds in my twenties. The first stage is to listen, listen and listen until you can distinguish the sounds. Only then can you try to articulate them. It can take a long time, with continual practice, as you are using muscles in a new way, and you need to practice to build up the fine motor control associated with making the sounds, and using them in combination. In my experience most learners do not spend anywhere enough time mastering, or approximating, the target language sounds. I suspect traditional classroom teaching is part of the reason.
In truth a person does not
need to exactly reproduce all sounds, as long as they are understood. (A Russian Lithuanian friend of mine is very hard to understand due to his accent.) And I bet German speakers of Esperanto carry over their stress timing, whereas French speakers carry over their syllable timing. English speakers will tend to carry over dipthongs, whereas Italians will tend to use pure vowels. And of course the verb conjugations are new to Chinese speakers and anyone else whose language lacks such a concept.
The "dividing line" between different vowel phonemes is different in every language (and every dialect). That is a problem for people learning a new language. If my language uses 2 sounds as different phonemes, and they are the same phoneme in your language, they will sound "identical" or "almost identical" to you. In reality, they sound like "the same phoneme" to you. We are all used to a single "phoneme" using a variety of sounds.
A very common example is Spanish speakers of English. Spanish has the /i/ sound but not the /ɪ/ sound. So to a Spanish speaker, the words "hit, bit, sit, fit" sound the same as "heat, beat, seat, feet". This is true about anyone learning a foreign language. Chinese has different phonemes that I cannot distinguish. English is that it has more vowel phonemes than many languages. But I think that learning Cantonese, or Thai, or Hindi will present the same problem to you.
I think the main problem with English is not the sounds, but the orthography which must be a total nightmare. Welsh, despite its reputation, is so simple to pronounce compared to English.
In my opinion artificial languages are pointless, yes even Klingon, simply because they lack a culture, and a clear purpose for learning. And they will always benefit one group of speakers eg Indo European. A huge number of people speak English as a second language because America is the dominant economic and cultural force today, and English has become the lingua franca for business and science. I remember speaking to some Belgians at a scientific conference, and being astonished that they spoke Flemish and English, but not Walloon. Like Greek and Latin, English will eventually cede its role to another language, such as Mandarin.