You're right, the word "acquaintance" sounds very impersonal to English ears. For a more neutral tone, we might use "(s)he's someone I know... (from work/from school/through a friend/etc)"

It's kind of outdated and archaic nowadays, people would just say "someone I know of", but people from high society may still use it.

I agree with Sarah. The meaning is more impersonal in English. We use it for someone we have met but don’t really know or wouldn’t consider a friend.

The reason "why" is that word usage frequency varies from language to language. Just because there is a direct translation of one word into another doesn't mean that both words are used with the same frequency in both languages.

English speakers across the globe have different ways to say things. In general your observation is correct that we do not use the word "acquaintance" as much. Using it, however, is not a problem as we should all understand it and don't really see it as meaning anything more than just that. So go ahead and call your acquaintances, acquaintances. no one will think you mean anything impersonal by it. There are a couple reasons why this has happened. Keep in mind that this is just my own theory... 1) English speakers like to use informal language that is localized. We here in NZ say "Cheers" instead of "thank you" but you'll still hear both and no one will think anything of hearing "Thank you" instead of "Cheers". 2) Our education system might be the issue too. In the southern hemisphere of the world we don't learn such words, or use them as kids. And because of that, as soon as we learn this word it's probably too late to get used to using it as we've already gotten used to using other ways to express the same thing. I think that's the main reasons this has happened.