Add GetCultureByAcceptLanguage extension for HttpContext

Introduced a new extension method, GetCultureByAcceptLanguage, in the WebExtensions class. This method parses the Accept-Language header from the HTTP request and returns the first valid CultureInfo, enabling detection of the user's preferred culture from browser settings.

EnvelopeGenerator.Web/Extensions/WebExtensions.cs

@@ -3,8 +3,10 @@ using EnvelopeGenerator.Application.Common.Dto.Receiver;
 using EnvelopeGenerator.Web.Models;
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authentication.Cookies;
+using Microsoft.AspNetCore.Http.HttpResults;
 using Microsoft.AspNetCore.Localization;
 using Microsoft.AspNetCore.Mvc;
+using System.Globalization;
 using System.Security.Claims;
 
 namespace EnvelopeGenerator.Web.Extensions;
@@ -116,4 +118,31 @@ public static class WebExtensions
         Body = "Bitte kontaktieren Sie das IT-Team."
     });
     #endregion
+
+    #region HttpContext
+    public static CultureInfo? GetCultureByAcceptLanguage(this HttpContext context)
+    {
+        var acceptLanguage = context.Request.Headers.AcceptLanguage.ToString();
+        if (string.IsNullOrWhiteSpace(acceptLanguage))
+            return null;
+
+        foreach (var value in acceptLanguage.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
+        {
+            var cultureName = value.Split(';', 2)[0];
+            if (string.IsNullOrWhiteSpace(cultureName))
+                continue;
+
+            try
+            {
+                return new CultureInfo(cultureName);
+            }
+            catch (CultureNotFoundException)
+            {
+                // ignore invalid cultures and continue
+            }
+        }
+
+        return null;
+    }
+    #endregion
 }