Is there a way to change the WHMCS language programmaticaly, even with the use of hooks?
WHMCS API doesn't have a function to change the language. Therefore, your best bet would be to use an action hook, and when that hook executes, you would need to change the language by directly interacting with the database. You would directly use a mysql_query() function, since the connection is already established by WHMCS itself.