Within Accounts receivable parameters > Update > Invoice there is a parameter called "Default language for sales invoices". This parameter can be set to "Order account" or "Invoice account".

When selecting "Order account" the invoice will always be printed in the language that is stored at the customer account that is currently selected within the sales order - that's fine.

When selecting "Invoice account" in the parameter then the invoice will always be printed in the language of the invoice account that is stored at the customer account at the time when the customer is selected. Though, it can occur that the invoice account will get changed on purpose within the sales order. The new invoice account might have a different language stored. But the invoice is not printed in the deviating language.

This leads to the fact that in these cases the invoice will be printed in a language that the invoice account doesn't speak, neither maybe does the customer account. So, none of them would understand the invoice properly.

The idea is that by setting this parameter to "Invoice account" the system will always consider the invoice account, that is currently selected in the sales order, and not the default invoice account language of the customer account.