You need to have a French bank account to be paid your salary, even if you already have a Euro account from another country (actually there seems to be counter-examples to this…). The quickest way to set up an account is either to deal with one of the two banks at the CEA Saclay campus (who are used to dealing with international applicants) or via a branch of Societe Generale (who have a special scheme arranged with Fnak for foreign researchers). Other banks may create hurdles, such as demanding 3 months of payslips (which is obviously impossible). You need to set up an appointment to create an account. The waiting time depends on the bank.
Science Accueil can set up your bank account before you arrive, and there is only need to come to sign upon arrival.
The French state has put together a website (in French) which allows one to compare banking fees of all known banks: https://www.tarifs-bancaires.gouv.fr/
Bank branches at CEA Saclay:
Others: