You would need to have all the article be in the same category. The MediaWiki software doesn't recognize articles in subcategories to be part of the higher categories.
I would recommend a specific backup category for producing XML backups.
I actually created a specific system of create boxes and preload templates for accomplishing this. You can get it here:http://hub.wikkii.com/wiki/Wikkii_Backup_System
However, in order to make proper backups, you should get advanced hosting so that you will be able to backup full article histories more easily, all articles without worrying about whether they're all in the backup category, and be able to backup user accounts as well as deleted pages.
You will also be able to set the wiki language with advanced hosting.
You can also add your wiki to the Community Wiki (link in my signature). I'm working on getting all wikis and wiki users connected to help improve membership among different wikis and help improve the overall experience of the users here.