As to a bare-bones DLL "cracker", have used this tool in the past (in days of re-skinning XP). As to an all-in-one plain language dll decrypter?? Dream on!!
To break down a DLL file, you need to have at least a basic understanding of the type of DLL file you are trying to unravel. Just because they use a common file-type, don't assume they are all the same... as they are not. Apart from the fact that I've never found a tool that is even capable of unravelling every DLL file thrown at it, to expect a "plain-text" interpretation of every such file is unrealistic. Know the basic context of the DLL file you are trying to tackle, then Google for a specific tool to meet your ends... there are many to be found :)
A dll file contains relocatable binary data/code that can run in a Windows environment. There may be readable strings contained in it, but most of the data is machine code of some kind. As others have suggested, there are cracker and decompiler tools that can turn the binary code into a more readable form, though it will not necessarily be "human readable" unless you are an accomplished assembly language programmer.
So, my question to you is this (in 2 parts) - why do you need to do this, and what do you need to accomplish by doing this?
They are pretty much the same question, but with different intentions.