"The Polish cities are beside rivers."
Translation:A lengyel városok folyók mellett vannak.
From Proto-Slavic "*lęděninъ" - dweller of wasteland / wold, virgin soil.
In a previous example is was not ok to leave out the article, even when the english translation did not include 'the'. Here, you not accepting a version that includes the hungarian article 'a'. At least you should explain when a general statement requires the use of the article and when it does not.
"-nál" and "mellett" aren't the same. Honestly, for me, this sentence doesn't sound great either way but generally, "mellett" is much more exact about the location - "next to", "beside", by somethings side. "-nál" is much more like "at" or "by". Somewhere around a point.
Having said that, one might find both okay here (I'd prefer "... folyóparton vannak" or something like that anyway) so it may get added eventually. However, the difference between "-nál/-nél" and "mellett" is important so I thought it was worth mentioning.