This article explains how to create a FreeAgent invoice in a foreign language.
The language setting is under 'Invoicing Options' for the contact. Choose what language you want to invoice that contact in.
You can choose the currency when you create the invoice itself, but the language option must be set at the contact stage.
Then when an invoice is created for that contact, it will be in the foreign language.
Please note that FreeAgent doesn't translate the description you type in, so if you want that to be in the foreign language, you will need to type it in in that language.
The invoice is then ready for sending as usual.