1) Localizing content language(for web site visitors) and localizing editor UI language (for content editors). Are these two implemented differently in Episerver?

There are two ways to localize. 

2.1) Xml

What I do not understand is how to get the localized string when there are multiple languages mentioned here?

2.2) Localization service API

This is clear. But can this be used in localizing editor UI language (for content editors)?

The example for #2.1 

[ContentType(
    Description = "This text can you have in XML instead"
)]
public class StandardPage : PageData
{
        [Display(
            GroupName = SystemTabNames.Content,
            Name = "This text can you have in XML instead ",
            Description = " This text can you have in XML instead " 
        )]
        public virtual ContentArea MainContentArea { get; set; }	
}

  
    
      A description of the page type
      
        
          Name text from XML
          Description text from XML