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Generally, I would use the .fr tld for a site in French, .net or .com for a site in English.
If you use both French and English on your site, I would use the .fr tld for the French part and .net or .com for the English part.
As you certainly know, all major international companies use a .fr tld to promote their products/services in France.
The site will be mainly geared toward french people - I will possibly have the one for the worldwide market market as well but first it's going to be french only.
What is more popular with french people - let say I do some advertising - if they only remember the domain name and not the tld will they type .fr or .com.
If the .com is typed more often I might as well try and find a new domain name - although I REALLY like the ones (.net & .fr) I have at the moment.