To change the language Hotaru CMS uses, you need a language pack. You can download language packs from the Language Pack forum.
1. If you don't have a "languages" in your theme folder, you need to make one, i.e.
/content/themes/YOUR_THEME/languages
2. Unzip italia.zip (for example)
3. Copy all the files from /italia into your new languages folder, e.g.
/content/themes/YOUR_THEME/languages/main_language.php /content/themes/YOUR_THEME/languages/admin_language.php /content/themes/YOUR_THEME/languages/bookmarking_language.php ...