1) Download packages from the following page
http://www.phpbb.com/downloads.php
Two files: language files and skin files
http://prdownloads.sourceforge.net/phpbb/lang_chinese_simplified.tar.gz
http://prdownloads.sourceforge.net/phpbb/subSilver_chinese_simplified.tar.gz
2) Uncompress the language file
$ tar -xvzf lang_chinese_simplified.tar.gz
$ tar -xvzf subSilver_chinese_simplified.tar.gz
3) Install the language files
$ cp -R lang_chinese_simplified phpBB2/language
$ cp -R /tmp/subSilver/images/lang_chinese_simplified templates/subSilver/images/