Well, if you become an expert in one language, you automatically become average at other languages. But, first you have to be an expert in one language.
Programming is much like driving. If you learn to drive one car, the other vehicles are then easy to learn. At the end, it is about how well you can drive on a road and avoid accidents: translated to programming terms means: how well you can write efficient code while saving yourself form serious bugs.
And according to me, be an expert doesn't mean you just know one language to its full extent. Rather, I think it should mean that you can write better code in that language. You should be good at naming variables ( it is the most difficult part of software engineering ), should be good at designing classes, should be good at recognizing Design Patters, when and where to apply, and even when not to apply Design Patterns.
I started with Java, went to PHP, and now doing Ruby. It takes a very short time at getting familiar with the syntax of other languages, but at the end of the day, it is about how much readable and maintainable your code is.