I've noticed this within accents of the same language. My own English accent has a very mild separation between the i and e vowels in, e.g., bitter and better. To my ear they sound wildly different, but many native speakers from other places and more English-as-a-second-language speakers can really struggle to tell apart what I'm saying, to the point that it's usually easier for me to switch languages to the local one to distinguish the words if the context is ambiguous. Oddly, despite noticing shifts in my accent having lived abroad for some time now, this is something that hasn't budged.

Similarly in my (learnt as an adult) second language, there are a couple of vowel sounds that aren't in English and I usually have to really focus to hear them, and to pronounce them correctly.