You can't return a key value pair, or a list of key value pairs from a method that is set up to return a list of strings - C# is strongly typed, and checks parameter types!
You can change the method return a list of key value pairs:
List<KeyValuePair<string, string>> GetLanguagesForMobile()
{
...
}
Or a Dictionary:
Dictionary<string, string> GetLanguagesForMobile()
{
...
}
And recode the method to generate the appropriate data - but if you declare a method as returning a type, you can only return that type or types derived from it.