Supported languages (43 and counting):

Albanian, Arabic, Belarusian, Bosnian, Bulgarian, Catalan, Chinese (Simplified), Chinese (Traditional), Czech, Danish, Dutch, Estonian, Finnish, French, Frisian, Gaeilge, Galician, Georgian, German, Greek, Hebrew, Hungarian, Icelandic, Italian, Japanese, Korean, Latvian, Norwegian, Norwegian Nynorsk, Polish, Portuguese (Brazil), Portuguese (Portugal), Romanian, Russian, Slovak, Slovenian, Spanish, Swedish, Taiwan, Thai, Turkish, Valencian, Vietnamese 

To install the langpack, simply copy the file (utorrent.lng) into directory with the µTorrent executable, then restart the client. It will install the langpack automatically.