One more interesting question !
It's the same as “faire la bise” or serrer la main”.
As a foreigner, use the polite form “vous”.
If someone thinks it's time to switch from “vous” to “tu”, he/she will start talking to you using “tu”, or he/she will say something like :
“on peut se tutoyer maintenant”
“on pourrait se tutoyer”
“on pourrait se dire tu”
Be careful, if this person is old, or if it's your boss, keep on using “vous” until he/she tells you to switch. Do this, because in France, in some cases, someone can use “tu” talking to someone younger for example, but the other person must use “vous” to show his respect. And it's only this older or important person who can decide if you can use “tu” and when you can use it.
When this person decides you can use “tu”, he will say something like :
“tu peux me tutoyer”,
“tu peux me dire tu”.
In fact it's better to be "too polite" than unpolite. If you're not sure, keep on using "vous" untill you are told to switch.
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