The user and all related content has been deleted.

1. Probably, but these are the official locale names since there are several "de" variants as there are "en" ones.

2. Zenphoto does not require the locale in the url as it stores the current one for the current visitor in sessions/cookies. If you use the sitemap in multilingual mode it generates each url with all locales you have activated. You should add the link to the sitemap ot the robot.txt file. All proper search engines should use that then.

3. That is an option of the dynamic locale plugin. You should have seen that at the bottom of the setup script when installing.

The user and all related content has been deleted.

The locale is not removed from the URL. It is just not put there in the first place since it is not needed.