I might be wrong but the locale settings is not something you change in your code.
This is determined by the browser's user configuration.
If you are using an HttpClient you definitely send the value you want for Accept-Language header to the server, but don't expect to change this from your code and make it effective in ALL the requests originated in the browser.

Common practice is to set a custom cookie with your user's language preference. By default you should populate this cookie with the value on Accept-Language header.
In ASP.NET MVC there is a setting so both Thread.CurrentThread.CurrentCulture and Thread.CurrentThread.CurrentUICulture get's set according to the user's browser configuration:

<system.web>

    <globalization enableClientBasedCulture=true uiCulture=auto culture=auto />

</system.web>

You can then create an ActionFilter that would check the custom language cookie and will set the Thread.CurrentThread.CurrentUICulture accordingly.

Thread.CurrentThread.CurrentUICulture is used to determine which resources the code should use. While Thread.CurrentThread.CurrentCulture is used for all the numeric and dates handling.

This is how that action filter might look like:

public class LanguageActionFilter : IActionFilter

{

    public void OnActionExecuted(ActionExecutedContext filterContext)

    {

    }



    public void OnActionExecuting(ActionExecutingContext filterContext)

    {

        var langCookie = GetOrSetLanguageCookie(filterContext.HttpContext);

        var culture = new CultureInfo(langCookie.Value);



        Thread.CurrentThread.CurrentUICulture = culture;

    }



    private HttpCookie GetOrSetLanguageCookie(HttpContextBase context)

    {

        var cookie = context.Request.Cookies[Constants.LanguageCookieName];

        if (cookie == null)

        {

            //set the cookie for first time

            cookie = new HttpCookie(Constants.LanguageCookieName, Thread.CurrentThread.CurrentUICulture.Name)

            {

                Expires = DateTime.Now.AddYears(1),

                Path = /

            };

            context.Response.Cookies.Add(cookie);

        }

        return cookie;

    }

}

Pleae Note:

• Even when your user selects, let's say es-Ar in your interface, and you set your UICulture accordingly for that user, IF the user's browser is configured with en-US dates posted for that user will be parse using the English convention, so float number will.

IE: If the user in USA with a browser set to en_US chooses to use the Spanish interface of your app, when this user post information to the server dates like:

1/5/2015 will be parse as January 5th in the server, rather than May 1st.

Numbers like 1,900 will be parsed as 1900 in the server rather than 1.900.

I learned this the hard way.
I have a question here that was never answered.

Please let me know if you have any questions.