Get browser language in .AspNetCore2.0?

IRequestCultureFeature provides the first matched language, which is supported by your application. Declaration of supported languages is defined in Configure() of your Startup class (see example). If you still need all accepted languages as a simple string[] like the older Request.UserLanguages property, then use the HeaderDictionaryTypeExtensions.GetTypedHeaders() extension defined in the Microsoft.AspNetCore.Http namespace:

// In your action method.
var languages = Request.GetTypedHeaders()
                       .AcceptLanguage
                       ?.OrderByDescending(x => x.Quality ?? 1) // Quality defines priority from 0 to 1, where 1 is the highest.
                       .Select(x => x.Value.ToString())
                       .ToArray() ?? Array.Empty<string>();

The array languages contains the list of accepted languages according to the priority parameter q. The language with the highest priority comes first. To get the default language take the first element of the array languages.

As an extension method:

using System.Collections.Generic;
using System.Linq;

using Microsoft.AspNetCore.Http;

public static class HttpRequestExtensions
{
    public static string[] GetUserLanguages(this HttpRequest request)
    {
        return request.GetTypedHeaders()
            .AcceptLanguage
            ?.OrderByDescending(x => x.Quality ?? 1)
            .Select(x => x.Value.ToString())
            .ToArray() ?? Array.Empty<string>();
    }
}

You need to add the localisation middleware to be able to get the IRequestCultureFeature feature:

public void Configure(IApplicationBuilder app)
{
    //...

    //Add this:
    app.UseRequestLocalization();

    //...
}

Now in your controller you can request the feature like this:

var requestCulture = Request.HttpContext.Features.Get<IRequestCultureFeature>();

You can get the browser language from the Request Headers

Write on your controller:

//For example --> browserLang = 'en-US'
var browserLang= Request.Headers["Accept-Language"].ToString().Split(";").FirstOrDefault()?.Split(",").FirstOrDefault();