Unfortunately, we do not have anyone in staff speaking Slovak, thus I hope a reply in English will be of some help.
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 Slovak 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 Slovak.
But there is no way to have the invoice that you print from Ecwid control panel in Slovak at the moment, because we have no Slovak translation for Ecwid backend. If any Slovak-speaking customers could provide us with one, that would be great.