I agree with hermarphromoose #7 and CM2DD #13 (the only identifiable native speakers who have posted in this thread).
In German (and maybe in other languages), 'Du' is used very differently, just to get someone's attention or as a substitute for their name. That's very different from English, where 'you' is not normally used in direct address.
'Hi you' is not a common or normal greeting, either in conversation or in an e-mail. More than anything else, it sounds unidiomatic, so it doesn't surprise me to hear that it is used in spam written in broken English. (Though I wonder who actually reads spam.) If a native speaker did use it, it could indeed easily be flirty or intimate, but that would probably be between two people who were already very close, so in your context it just sounds out of place.
However, I doubt that that it's fair to blame an interpersonal misunderstanding on a language problem alone. After all, it's clear that someone who is not a native speaker will not always choose the right word.
Things like this just happen, even between people in the same culture. Person A likes Person B and would like to get to know him or her better, maybe go out, see if there's any basis for a relationship. (To me that's still well short of a Liebeserklärung; it doesn't have to be that serious.) But in this case, Person B just isn't that interested in Person A.
Both of those positions are entirely reasonable. It's tough for Person A (often the guy) to get up the nerve to say something and risk rejection, but reasonable to try; if no one ever asked, no one would ever get together. But it's tricky to find the right words to show interest without seeming too pushy, and it doesn't always work.
It's tough for Person B (often the girl) to have to respond, but no one can force anyone to feel an interest they don't feel. The polite thing might be to go out at least once, just to dinner or something, but if there's clearly no future in it, then it's reasonable to communicate that. But it's tricky to find the right words to be kind but firm, and it doesn't always work.
That's why friendship and dating are, well, hard. Language problems probably complicate the issue somewhat further, but it's complicated enough already.