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?
Most scripting languages can handle this kind of thing (by running external programs written in other languages,) but it seems as if your best bet might be shell scripting of some kind (Windows users call this "batch scripting," but the DOS syntax is horrible and not recommended.) UNIX programmers have been freely mixing languages in this way for a long time. On Windows, you can install Cygwin to get a fully functional BASH shell. 
The shell was originally intended as a user interface, used to launch other programs or combine them in interesting ways. However, many shells (notably the Bourne shell or its modern descendant, BASH) are also fully-fledged programming languages. Each of your "modules" can be created as separate, standalone programs which will be run by the shell script.
Which dynamic language can easily use libraries from other languages?