Regional settings are not real regional settings, because MediaWiki doesn't have a concept of region. It only has languages.
[edit] Time and date formats
- Time and dates are shown on special pages and alike. The default time and date format is used for signatures, so it should be the most used and most widely understood format for users of that language. Also anonymous users see the default format. Registered users can choose other formats from the preferences. If you are familiar with PHP's time() format, you can try to construct formats yourself. MediaWiki uses similar format string, with some extra features. If you don't understand the previous sentence, that's ok. You can provide a list of examples for developers. Be prepared to answer questions about minor details.
[edit] Number format
TODO: write something