> If I bought Intellij IDEA, does that mean I can download free plugins to get the same functionality as PyCharm?

Yup, pretty much yes.

> Does PyCharm support Javascript? If I want to do Django and Javascript, do I need to buy IDEA?

PyCharm does support js so you don't need IDEA unless you target jython.

IntelliJ IDEA Ultimate brings you nearly all the feature that are available in the other version. PyCharm, WebStorm and others are cheaper but more focused on a single language.

as it was commented below IntelliJ IDEA includes almost the same support (mostly the same) for other languages (PHP/Ruby/Python) as other IntelliJ-based IDEs. But at the same time IntelliJ IDEA never was positioned as a "true" polyglot IDE. IntelliJ IDEA is Java platform IDE, in the first place. It does support other technologies via plugins though, it might be much more convenient to use dedicated IDEs for other technologies.

> Does PyCharm support Javascript?

Yes, to the point where I have edited in PyCharm even when I wasn't doing anything with Python, just for the JS features.

As I understand it, the plugins give the exact functionality that the more language-specific products offer. However they are typically one version behind. e.g. if RubyMine is on v6, the ruby plugin for IDEA will have equivalent features of RubyMine v5

IntelliJ IDEA is released once a year, and smaller IDEs are about twice a year. Because of this the plugins may miss some features for some time, that's true. But now, after the release of IntelliJ IDEA they will be updated with the latest Ruby, Python, PHP, etc. functionality

I think IntelliJ IDEA has all the features of the other IDE products. Stuff are just added/removed by default and adjusted towards Java development