I followed the instructions on this page to install spell checking dictionaries.
If you have version 0.3 or later, you can use "Scripts -> Scripting TeXworks -> Show Scripts Folder" and navigate to the parent directory in the window that opens. (...) You should have a TeXworks resources directory in your "user profile" directory, typically C:\Documents and Settings\Your Name. This is automatically created the first time you run TeXworks, and some default resources are copied there. There should be a dictionaries subdirectory, but it is initially empty. Place the *.dic and *.aff files there, and then re-start TeXworks.
I let TeXworks open my scripts folder (which is C:\Users\Pieter\TeXworks\scripts), created a subdirectory called dictionaries and added the DIC/AFF files for en_US. After restarting TeXworks it still doesn't report any spell checking languages. What gives?