Translation:Last year we went to Poland and Slovakia.
No particular reason. Compare to French travel Aux Etas-Unis vs. en France.
Typically we say NA when we are talking about islands or at least peninsulas. Florida, Havaj, Island, Yucatan, Nový Zéland, Bali. But DO Anglie, Irska, Gronska.... Why Czech picked Slovakia to treat as an island? Mystery. Well, at least to me. But as you see above, the "island" rule does not really work too well. We treat Ukraine the same, though you could use both DO and NA with that one.