"...such a HUGE deal in the US..."

That's just too broad of a generalization. It might be among some groups, but certainly not everywhere or in all age groups or in all cultural settings. You just have to know where you are.... and.... just take chances. If unsure, people need to make the first attempt barely more than simple conversation. Responses will vary even within some groups.

You just can't define a rule-of-thumb that fits even the majority of situations.

(I have to confess.. I was 'flirted at' a couple of times many years ago and was was too dense to even realize it at the time. There's a fine line between too obvious and too vague. ☺)