Create a locale folder for the new language and translate the text strings.
Copy the flag image corresponding to your language into the locale folder for the new language.
Add a language
Add a new folder with the locale name (for example, sv-SE) to the locales folder.
Within that folder, add:
A flag image file called flag.xx-XX.png (where xx-XX is the name of the locale).
A language file called Locale.xx-XX.xaml that contains all the keys in the locale file of an existing language (it’s easiest to simply copy the locale file of another language and translate the strings in the file).
A language file called Locale.xx-XX.User.xaml which only contains the root wrapping XML element, and no string keys (this file will later be used to override strings in the default file).
Remove a language
Delete the folder under locales which belong to the specific locale you wish to remove.