Dell XPS laptops all have mixed function and media function keys. For the XPS 11, the special functions are turned on by default, with F-key functions requiring the Fn + F-key combination. This can be pretty annoying for those that need F-key functionality, such as for those using the language bar or Office.
To turn this off, either use the Windows Mobility Center (i.e. the customized by Dell section) or the computer BIOS. Read more at WindowsClub.
To access the BIOS, read more at Dell Support.
To read more about Dell special media key functions, read more here.