why not: slovakia isnt neighbour of germany?
You need an article before neighbour.
why nemecka here?
Another way to read the sentence is "Slovakia is not a neighbor OF Germany." So we have "Německa" (genitive) instead of "Německo" (nominative).
Why can't it be 'Germany is not Slovakia's neighbor?
Because it is s different sentence. It is about what Germany is or isn't while the original is about Slovakia.