Despite SQL Server 2005 supporting 33 languages (found by issuing SELECT * FROM sys.syslanguages), not all of these are available for the full text index service. To find out which ones are run the query:
SELECT *
FROM sys.fulltext_languages

On my machine, this returns the following languages:

  • British English
  • Chinese (Hong Kong SAR, PRC)
  • Chinese (Macau SAR)
  • Chinese (Singapore)
  • Simplified Chinese
  • Traditional Chinese
  • Dutch
  • English
  • French
  • German
  • Italian
  • Japanese
  • Korean
  • Neutral
  • Spanish
  • Swedish
  • Thai

An additional 6 languages are supported and available for a separate install. These are :
  • Danish
  • Polish
  • Portugu√™s (Brasil)
  • Portuguese
  • Russian
  • Turkish

To install these, follow the instructions here.
The following languages are not supported for full text searching at all within SQL Server 2005:
  • Arabic
  • Bulgarian
  • Croatian
  • Czech
  • Estonian
  • Finnish
  • Greek
  • Hungarian
  • Latvian
  • Lithuanian
  • Norwegian
  • Romanian
  • Slovak
  • Slovenian

SQL Server 2008 offers more full text language support bringing the total of available languages to 50. It would appear that Danish, Polish and Turkish remain installable additions.