Hi,
How to change the date format is described in this
FAQ.
Regarding your second issue, please follow these steps:
1) go to Site Manager -> Sites and edit your site
2) on General tab, there is a field called "Default content culture"
3) Click the "Change" button next to this field
4) Select desired culture, make sure that the check box in the dialog window is checked and hit OK
5) Now, all documents in your site are set to given culture.
Best regards,
Juraj Ondrus