Not sure what do you mean by "relatively" but teaching Prolog or a functional language to someone who's only seen C/C++ is, sometimes very, hard. I remember all those seasoned coders (or rather walking C code generators) unable to grok that a program doesn't have to be a list of instructions to follow, but might very well be just a detailed list of relations between the ... erm ... two or more things. Since Prolog predicates work both ways, you do not have input and output parameters. Prolog predicates define the relation between some X and Y so if you specify X, you get Y and if you specify Y you get X. So ... do you want to split a list in two or merge two lists to one ...

Just like learning French and Spanish will help fairly little in later learning Czech or Russian and even much less in learning Japanese, learning one or 1.5 languages will not necessarily help much with languages that are really different.