Go to your account > modules > scroll down until you see 'Invoices in other languages'. Activate the blue button.

Now create a new invoice.

Select an existing or new customer and create the invoice.

Scroll down to the 'Options' section where you can change the language. Press the 'Create Invoice' button.

Want to verify everything is correct? Go to 'More Actions' > 'Print Preview'. You will see that the language has been changed.

There you go, easy right?