Claws Mail's spell checker is based on the Enchant library, which uses OpenOffice dictionaries.
The official Claws Mail for Windows package ships with American English, French and German dictionaries.

You can get more dictionaries from the OpenOffice website and install them with the following procedure:

• Go to the OpenOffice dictionaries webpage.
• Click on the link for the dictionary you're interested in.
• Click the download button for the dictionary. You should be redirected and your download of a .oxt file start.
• OXT (Openoffice eXTension) files are really ZIP files, so rename your download to .zip.
• Open it, and extract the <language>_<country>.aff and <language>_<country>.dic files (for example, sk_SK.aff and sk_SK.dic).
  Note: Some OpenOffice dictionaries provide more than spell checking, and the zip file contains a lot of files that Claws Mail doesn't use. If the .aff and .dic files are not visible at the root of the zip file, they are usually in a subfolder named <language>_<country> (for example, sk_SK).
• Copy these two files (.aff and .dic) to Claws Mail's installation: %PROGRAM_FILES%\Claws Mail\share\enchant\hunspell\
• Restart Claws Mail. Your new dictionary should be available in Tools/Preferences/Spell checking.