modification of cre b2b 6.4 language option

Hi, I what you to make a modification of a cre b2b 6.4. shop. I what to be able to set which language should be default for the customer, this should be set from the admin from the edit/create customer page. When a customer logs into the shop, he should see the shop i the language defined in the above. Please ask questions before bidding. Regards Søren