>>Do they really use the french ''tu'' and ''vous'' in Hong Kong?
Do they know French at all?
I can't generalize at all, because a community of French learners here, for example, is hardly existent. Unlike quite a few parts of China, Hong Kong is more open (culturally; we're more exposed to foreign products, at least, and often have the money to buy) to foreign things, so there could be quite a few French learners, of which I know nothing about. My own experience is that, like some other langs, at least every year there are well over a few dozens of learners in two or three universities here who graduate. I believe, though, that a French major who reaches B1 before graduation should know the language quite well, so to speak. (I'm starting a new thread about this)
I am not quite sure if you got the difference in use between "Sie" and "du" in German right.
"Sie" is the first choice with people you do not know unless they are children. You would never use it nowadays with your relatives, such as uncles or parents or grandparents. Such a use of "Sie" for parents etc. is archaic, but you might of course find it in books if the story is in the past.
"Sie" and "Herr"/"Frau"+family name is also used for professors, teachers and other people you may have known for a while unless they offer you to change to "du". I.e., usually the senior (by age) persons offers to use "du" and usually the first name.
So "du" is for children, family, other students if you are a student at a college or university, and people who have explicitly told you to use "du".
It might get a bit complicated if, for instance, the much younger person has a higher "rank" in the institution you are with, i.e. if your boss is younger than you, who should offer to use "du"?
In many work environments someone (who knows why) has decided that all employees should use "du". I actually feel that is strange, perhaps influenced by English. Nevertheless, many of my colleagues also suggested that we could use "du" and our first names because they had already agreed to do so with some colleagues and wanted to avoid mixing up usages ("du" with person A and "Sie" with person B).
The parents of my friends, for instance, will always be "Sie" and "Herr"/"Frau" X to me, although I might have known them for just as long as I have known my friends. Now, if I see them a lot, many will after a while ask if it is ok to call me by my first name.
A weird kind of usage is "Sie" with the first name, which some secondary school teachers try to use as the pupils are growing up, say approaching 18. Most of them don't manage in the end. They are just too used to addressing pupils as "du" and using the first names. Some switch to "Sie" and the family names instead, which is just as strange, just because you are now 18.
So after listing some strange cases, as a rule I would suggest that you should use "Sie" in the first place with all adults, but not among students at university. People will tell you if they think it is ok to use "du".