Added translation fallback

. fallback 1: 
  if no entry or empty entry in user's language --> fallback to default language
. fallback 2:
  if empty entry at all (even in default langage) --> fallback to phrasename

application/models/system/Phrase_model.php

@@ -77,4 +77,15 @@ class Phrase_model extends DB_Model
 
 		return $this->execQuery($query, array($categories, $language));
 	}
-}
+
+	/**
+	 * Loads all categories 
+	 */
+	public function getCategories()
+	{
+		$query = 'SELECT DISTINCT category
+					FROM system.tbl_phrase';
+
+		return $this->execQuery($query);
+	}
+}