Get the correct locale from javascript in an ASP.net core application

I have an ASP.net Core version 5 application. In this application there is some javascript. In this javascript I sometimes need to convert between strings and numbers. This conversion needs to use the correct locale. I would like to use the same locale in my javascript as I'm using on the servers side. The server side locale is set up using a cookie with this code:

    Response.Cookies.Append(
        CookieRequestCultureProvider.DefaultCookieName,
        CookieRequestCultureProvider.MakeCookieValue(new RequestCulture(culture, culture)), 
        new CookieOptions { Expires = DateTimeOffset.UtcNow.AddYears(1) }
        );

I thought that I could set the locale on the client by setting ApplyCurrentCultureToResponseHeaders to true.

    var supportedCultures = new[] { "en-US", "en", "nb-NO", "sv-SE" };
    services.Configure<RequestLocalizationOptions>(options =>
    {
        options.DefaultRequestCulture = new RequestCulture("en-US");
        options.AddSupportedCultures(supportedCultures);
        options.AddSupportedUICultures(supportedCultures);
        options.ApplyCurrentCultureToResponseHeaders = true;
    });

This results in a response header called content-language being set to the correct culture. So the correct culture gets to the client. But now is the question, how do I use this in my javascript code? I'm using methods like toLocaleString and parseNumber when converting back and forth between number and string. The content-language header does not seem to change the values in navigator.language for example, which I maybe expected.

Or maybe I'm going about this the wrong way?