1. Photopost will give each file a unique numbering scheme meaning photo.jpg will become photo1.jpg photo2.jpg etc
2. Not sure what you mean locked but each category can be public or password protected and as well you may specify which usergroups can view upload or make comments to each cat so that will most likely be what your looking to do is refine permissions.
3. You can look at the english.php language file and make your own polish.php language file with the proper wording and charset meta and upload that file to your languages directory in photopost and in admin config options set the language to polish to load your new file. we will look for a php file with that name in the languages directory.
4. You can install in a separate database or the same database as vbulletin and we do integrate with your
vb forum and many examples can be seen on our site.
5. The license is forever meaning you never need to purchase a license again. However much the same as vbulletin you get one years members access to download upgrades and support and after this year totally optional to you that you can purchase members area renewals to gain access to new releases.