Tell why you think Python, Perl, Ruby, etc is easiest for plugging in modules from other languages with minimal thought.
To clarify, an example: I want to write business logic in Python, but use functionality that conveniently exists as a Perl module.
In other words, which language "just works" with the most modules?
If you want to plug in a Perl module, the language which is best suited for this is Perl. Perl is able to represent the semantics and capabilities of code written in Perl correctly. This really shouldn't be a shock.
If you have a self-contained program you want to call from another program in its own process, not constantly interacting, any of these languages can do that with programs written in whatever language. At that point, you aren't really using other languages inside a program but just calling other problems.
There are several projects to combine various pairs and projects (like Parrot) that seek to provide a platform for a large range of languages for compatibility and projects (like .NET) that almost accidentally provide compatibility among previously-incompatible languages. However, I do not think most of these are as robust, mature, and suited for combining normal code as you would hope.
Which dynamic language can easily use libraries from other languages?