Your train of thought is correct: it is important for a programmer to learn different languages. I would suggest Ruby, and Ruby on Rails.
Rails' great advantage is it makes it easy to do things the right way -- being able to pass your code to another developer without him spending weeks just to understand your code is pretty big deal.
The current tendency in a lot of languages is to put them 'on Rails', i.e. implement the MVC environment. This was done for Javascript, .NET and many others.
