It really depends on the language. Going from C# to Java and vice-versa is pretty seamless, but moving from Java to C++ can be dizzying even if the two have a lot of commonality.
In general, though, the more languages you learn the more likely it is that the next one you try to pick up is easier.
I posted earlier that it only takes me about two weeks to learn a "C-like language". But I never thought that was only the case because I learned C and then C++ first and used them for about 80% of my development for 10 years. If I had started out with C#, Java, or Python and then wanted to learn C or C++ it would have taken me a lot longer.
In general, though, the more languages you learn the more likely it is that the next one you try to pick up is easier.