As I understand it, translators are classed as profession libérale non reglementée, so you have to pay towards:
URSSAF Quite useful link to the "Cotisations en début d'activité"
Professions libérales pension scheme The link to "Régime de base" explains how much you have to pay.
RSI (Régime social des indépendants) Social security
For the first two years you pay a fixed charge, (around 3000? for the first year, iirc) based on a forfait,then the third year you pay a percentage based on previous income. During the third year they adjust what you've already paid, if it was too much / not enough. During the first two years, you have to pay the fixed amount even if you don't earn that much, and even after the first two years of activity there is a fixed minimum sum to pay (at least 1000?? I don't remember, even if you earn very little. TBH that is what is putting me off getting started as a freelancer!
However if you are in employment you can be exonerated from paying charges on your freelance income, depending on hours worked where. The info about that is on the RSI site.
I have tried to work out the taux de charges but I'm never 100% sure if I'm right because it's quite complicated. I think it's between 30 - 40%, it's the taux de retraite complémentaire which I'm not sure about.
Good luck with those sites, reading them made me lose the will to live lol
Here are a few other websites which might help:
APCE General info about setting up a business, including on getting started as a freelancer
The France forum on ProZ also has some useful info.
Good luck