"March is the month of the year."
Translation:Marzec jest miesiącem roku.
No, it translates exactly that way. First, Polish has no definite/indefinite articles, so there's only "month of year" left to translate.
But declination isn't there just to make your task more difficult. Other than making word order more liberal and making easier to extract the meaning if some other word wasn't heard exactly, sometimes it allows you to express something that would require a preposition in English. In this example "roku" is the genitive case of "rok". A common function of genitive is to tell, who is the owner of an object. In this example the year has a month, and just making year genitive is enough to express English preposition "of".