I agree with Jehane and as a French user, I'd be more at ease with the formal pronoun "vous" in both the official website and the wiki (and even in the blog).
Apart for casual blogging or live chat, I rarely use the informal pronoun "tu" which in my opinion implies relational proximity or at least implies that you know your reader.

I'd prefer to use "vous" on the wiki, which doesn't exclude informal writing in French (we use it on the Mageia blog for example).

It seems that many languages have this distinction between formal and informal pronouns, but with relative uses. I think that in Spanish for example "usted" (the formal one) is less used than "vous" in French.

Do we need to decide something for all the teams or is the choice left to the teams, i.e. does it need a global policy?

Cheers,
Rémi