Unfortunately, Niche Raider is not "translation friendly". In order to translate to another language, you would need to translate most of the theme and plugin files.
Here are some helpful links:
http://codex.wordpress.org/Installing_WordPress_in_Your_Language
http://www.solostream.com/blog/tutorials/translate-wordpress-theme/
You can also do a Google search for WP plugins that can do that but again we strongly recommend you to make a back-up just in case something goes wrong.