Hi Micha,

Using a real factura from a Spanish company for reference I will list you the proper translations for some incorrect fields:
Issue date - Fecha de factura
Invoice No. - N° de factura
Payment type - Método de pago
Total net price - Subtotal sin IVA (Base imponible)

The modifications are not too dramatic so it would be best if you allow customisation of the translations for each user in the backend.

Thank you