Hello;
simply edit lang/your_language/main.php.
Make needed changes here:
$lang['categories'] = "Categories";
$lang['sub_categories'] = "Subcategories";
and any where you want to switch from "categories" or "category" to "artists" or "artist":
//-----------------------------------------------------
//--- Categories --------------------------------------
//-----------------------------------------------------
$lang['no_categories'] = "No categories found.";
$lang['no_images'] = "There are no images in this category.";
$lang['select_category'] = "Select category";
$lang['site_stats'] = "<b>{total_images}</b> images in <b>{total_categories}</b> categories.";