For Windows, dictionaries listed on the OpenOffice wiki are no longer available. They link to an FTP server that is now offline.
Instead of linking to the wiki, you should link here: http://extensions.services.openoffice.org/en/dictionaries
Also, it would be helpful to include these instructions:
1. Select your dictionary and download it. The file extension will be .oxt, but is really a zip file. You can rename it to a .zip extension or open it with 7-zip.
2. Extract the .oxt file to a directory. You may want to place it in Geany's etc directory (ie: C:\Program Files (x86)\Geany\etc).
3. Once extracted, go into Geany's Plugin Manager, then Spell Checker preferences. You will have to enable the Spell Checker plugin and set the path to your extracted dictionary.