Do you mean the Invoice which your customers can print in the store front-end or the one you can print in control panel?
If you enable Swedish language for your store (Ecwid control panel → System Settings → General → Languages ), the printable invoice that your customer can print from your storefront after placing an order, will also be in Swedish. You can change the text for the printable invoices you customers get by means of the Ecwid Translate Tool
But there is no way to have the invoice that you print from Ecwid control panel in Swedish at the moment, because we have no Swedish translation for Ecwid backend. It is not possible to add a translation for some part od the backend for just one Ecwid store. Backend translations are added globally, i.e. when such a translation is added, it can be applied to all Ecwid stores. If any Swedish-speaking customers could provide us with one, that would be great.