You have a contract that was already created for some Customer. Now you have changed the language for this account and when you execute a Preview, the contract is printed with the old language.
When the Customer in entered in a Contract, the language from the customer is copied over to the Contract. Later then this copied language is taken for Preview Form.
Changes on Customer Master will have no effect for the existing contracts.
A solution for this case could be to remove the Customer from the Contract and directly add it again, then the new language should be taken over.
KBA , LOD-CRM-EMP , Employee , How To