yes and yes

pierre, your questions are a bit vague. I suggest that you start by reading the documentation here:

http://docs.elgg.org/

and post more specific questions after that.

A hint for non-English writers:

The answer to the English question "Can Elgg do X?" is always "yes" if X is something that a computer can be programmed to do.

It is much better to ask "What is the best way to get Elgg to do X?". but only after reading the documentation and googling for "Elgg X" to see if your question has already been answered.

Just to be clear, I interpreted the question to be about licensing rather than a technical question.

Ok
I'll try to be clearer,
My project is to create a CMS based on Elgg but this time not for the social network but to create a dating site.
Because I know there are a lot of webmaster seeking this kind of script but there are very few or they are expensive

N.B. Sorry for my poor English

Non désolé pour les pauvres en anglais ;-)
Utilisez Google pour traduire ;-O

Of course... Elgg can be customized to create a "dating" web-site. There's many GPL PlugIns available for free and yes - some special custom commercial PlugIns might need to be created to launch your dating elgg-site.