Allow Visitors to Change the Language of Pages
WHMCS includes a number of translations for the public and client facing areas. You can allow customers to choose to view the site in their own language.
Navigate to Configuration > System Settings > General Settings (Setup > General Settings prior to WHMCS 8.0) and choose the Localization tab.
Check the Enable Language Menu checkbox.
Click Save Changes.
Visitors will now see a button in the bottom-right corner of the page that they can use to select their preferred language.