If you have any Windows Phone 7 apps from the original templates, unlike the Windows Phone 8 app templates they did not come with localization setup already. So lazy me just wrote the strings inline. Fortunately adding localization after updating to Windows Phone 8 is easy.

1. Add a Resources folder
2. Add new Resources resx to the folder named AppResources.resx
   
3. Ensure Custom Tool is set to PublicResXFileCodeGenerator
   
4. Add a new class named LocalizedStrings to the root of your project
   

   using YourAppNamespace.Resources;
   
   namespace YourAppNamespace
   {
       /// <summary>
       /// Provides access to string resources.
       /// </summary>
       public class LocalizedStrings
       {
           private static AppResources _localizedResources = new AppResources();
   
           public AppResources LocalizedResources { get { return _localizedResources; } }
       }
   }

5. Add an Application.Resource of your LocalizedStrings class to your App.xaml

       <Application.Resources>
           <local:LocalizedStrings xmlns:local="clr-namespace:YourAppNamespace" x:Key="LocalizedStrings"/>
       </Application.Resources>

6. Add strings in AppResources.resx

   

7. Use localized strings anywhere in your Xaml
   

   CompanyName="{Binding Path=LocalizedResources.CompanyName, Source={StaticResource LocalizedStrings}}"

8. To Add other languages and for more details follow How to build a localized app for Windows Phone