If you are developing a web application, you can use the Accept-Language Http header to define the language
EDIT 1
For a winforms application, you can use System.Globalization.CultureInfo.CurrentCulture.Name .
EDIT2
To get locale using the FB REST API:
 dynamic fbResult = new Uri("https://graph.facebook.com/AngelaMerkel?access_token=AAABkECTD......").GetDynamicJsonObject(); Console.WriteLine( fbResult.locale ?? "-" + " > " + //<---- fbResult.location.country + " " + //<---- fbResult.location.city + " " + //<---- fbResult.name + " " + fbResult.gender + " " + fbResult.link + " " + fbResult.updated_time); 
You can find information about my GetDynamicJsonObject extension method here
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