(TexWorks 2.8 on Windows XP)
1.- Obtaining dictionaries
There is a list of available dictionaries at:
http://extensions.services.openoffice.org/en/dictionaries
TeXworks can only use the spelling dictionaries (*.dic and *.aff) for each language;
To get them, rename the downloaded .oxt file to .zip and extract the spelling dictionary files (they may be contained in a subdirectory).
2.- Installing
To find the correct folder, use "Help -> Settings and Resources...", navigate to the "Resources" folder given there, and finally put the spelling dictionaries in the dictionaries folder (if it doesn't exist, create it).
By default, the TeXworks resources directory should be located 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
(Taken from http://code.google.com/p/texworks/wiki/SpellingDictionaries)
