So here is the context: Suppose as a tourist one is helped by someone (not a local though), talking to the locals to get the right directions for me etc. Chances of meeting again seem negligible unless huge (and I mean really huge) coincidences happen. In the end while separating, she says "à bientôt". Wouldn't "au revoir" been more relevant? What is the difference between the two?
Your are right "Au revoir" is the good way to leave somebody you will not meet again.
"À bientôt" may be a habit, or a try to encourage the foreigner to come back again in France!
With the latter you hope you will see the person again within a short period of time. With the former, the lapse period is simply indefinite.
It's more about how soon you expect to meet that person again, and how close you are to the person (if you're close, you'll probably meet again soon, though).
Au revoir litterally means "until next time we meet".
So, in order of increasing time span:
A tout de suite: see you in a bit, for example when you separate, usually under an hour
à tout à l'heure: within the day
à plus tard: see you later
à plus: undefined time span
à bientôt: other meeting greatly expected, sooner or later, kind way to say goodbye
au revoir: more formal, more distant than "à bientôt"
adieu: farewell, might be linked to "à dieu", meaning "at god", or when we'll be at god's side.
- casse toi pov con: presidential way to say goodbye to an old man who doesn't want to shake your hand
Now you bring this question, i'm wondering if there are so many ways to say goodbye in other languages.
Bientot means soon; A, in this case, is used in the sense of until, rather than "to" - in effect, a shortening of jusqu'a. So the implication is "until soon"
Au revoir utilizes the same sense of A - and revoir is voir - "to see" and re - again, so revoir is to see again. A bit more abstract and indefinite - "until the seeing again" or until we meet again - whenever that might be.
A bientot would be used in the case of, say, I'll see you after lunch, whereas au revoir might be used when seeing someone off at an airport on a trip.
Finally, adieu is, in essence, the analog of vaya con dios - go with God - and would be used when it is not at all clear that you will meet again.