Translation:Is Holland the same as the Netherlands?
That's because it really is complicated. The main part to remember is that Holland are two provinces (Noord-Holland and Zuid-Holland) and the Netherlands is the whole country (all twelve provinces).
Polish is not the only language to do so…:( I wouldn't be surprised if variations on Holland are more common than variations on Nederland or low country in most languages. At least our neighbours got it right, Niederlande in German and Pays-Bas in French, now only to teach English speakers to use the Netherlands, and then all other languages. :)
In Portuguese, it's way more common to hear "Holanda" than "Países Baixos" (literally: "Low Countries"), unfortunately. Actually, most people use to think that "Países Baixos" concerns more than one country (usually the Netherlands, Belgium and Luxembourg), since the name comes in a plural form. This misconception is quite difficult to change. :(
Further complication/possibly clarification:
In the Netherlands we make the following distinction: Holland is the two provinces of Noord-Holland and Zuid-Holland. Nederland is the name of our country. De lage landen (the low lands) are the Netherlands, Belgium and Luxembourg put together.
So, when in your language "Nederland" is called "the low lands", both that and the group of countries would be correct.
"The Netherlands" is plural because when it was first formed it was a group of provinces uniting (it still is), just like how america is called "The united states of America".