There is a certain order for determining the e-mail language. The order is the same as for other e-mail. If no specific customer language is imported from data, it ends up with the fallback language. It is possible to include a customer language from the customer import (fd_customer.xml) (see import definition for details). The following sequence is used to determine the e-mail language.
1. Email preference set from the customer account on the website (page "my preferences"). This setting allows the customer to set an e-mail preference. You can deactivate this option in the admin environment, from the module settings, my account pages tab.
2. Language selected on the frontend by clicking on the 'flag' to the right of the B2B.
3. Language sent along for the customer from the customer import. This concerns the field language_code (i.e. not country_code).
4. E-mail language from the active template. This can be found in the admin area, module templates.
5. Fallback language set from the admin environment in module settings, general tab.