It´s rather interesting, that the more languages you know, the easier it becomes knowing another one, especially if you know indoeuropean languages like latin or cl. greek(which i know), so learning i.e. french, italian becomes a lot easier if you know latin. Knowing danish help you to understand sweedish and norweigean, but also german because they are both germanic and thus have many words in commen, although the grammar is german is based on cases like in Latin with nominative, accusative, genitive ect.
The most difficult language i learned was cl. greek, mainly because it´s a dead language meaning that nobody speaks it anymore, but in neardy societies :d and thus you have to learn it academically through grammar et cetera whereas with a "living" language you could become better quite quickly, by talking the language with people, watching tv or hearing radio. That option doesn´t exist with dead languages, so it becomes a whole lot harder.