Redesign site as bilingual CV

Controllers/HomeController.cs

@@ -7,6 +7,7 @@ namespace LowLevelGuyCom.Controllers;
 public class HomeController : Controller
 {
     private readonly ILogger<HomeController> _logger;
+    private const string LanguageCookieName = "site-lang";
 
     public HomeController(ILogger<HomeController> logger)
     {
@@ -15,7 +16,25 @@ public class HomeController : Controller
 
     public IActionResult Index()
     {
-        return View();
+        return View(UseJapanese() ? "IndexJa" : "Index");
+    }
+
+    [Route("ja")]
+    public IActionResult IndexJa()
+    {
+        return Redirect("/?lang=ja");
+    }
+
+    [Route("cv")]
+    public IActionResult Cv()
+    {
+        return View(UseJapanese() ? "CvJa" : "Cv");
+    }
+
+    [Route("cv/ja")]
+    public IActionResult CvJa()
+    {
+        return Redirect("/cv?lang=ja");
     }
 
     public IActionResult Privacy()
@@ -28,4 +47,66 @@ public class HomeController : Controller
     {
         return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
     }
+
+    private bool UseJapanese()
+    {
+        string? queryLanguage = Request.Query["lang"].FirstOrDefault();
+        if (TryNormalizeLanguage(queryLanguage, out bool queryIsJapanese))
+        {
+            Response.Cookies.Append(
+                LanguageCookieName,
+                queryIsJapanese ? "ja" : "en",
+                new CookieOptions
+                {
+                    MaxAge = TimeSpan.FromDays(365),
+                    SameSite = SameSiteMode.Lax
+                });
+
+            return queryIsJapanese;
+        }
+
+        if (TryNormalizeLanguage(Request.Cookies[LanguageCookieName], out bool cookieIsJapanese))
+            return cookieIsJapanese;
+
+        return BrowserPrefersJapanese();
+    }
+
+    private static bool TryNormalizeLanguage(string? language, out bool isJapanese)
+    {
+        isJapanese = false;
+
+        if (string.IsNullOrWhiteSpace(language))
+            return false;
+
+        string normalized = language.Trim().ToLowerInvariant();
+        if (normalized.StartsWith("ja"))
+        {
+            isJapanese = true;
+            return true;
+        }
+
+        if (normalized.StartsWith("en"))
+        {
+            isJapanese = false;
+            return true;
+        }
+
+        return false;
+    }
+
+    private bool BrowserPrefersJapanese()
+    {
+        string acceptLanguage = Request.Headers.AcceptLanguage.ToString();
+        if (string.IsNullOrWhiteSpace(acceptLanguage))
+            return false;
+
+        foreach (string segment in acceptLanguage.Split(',', StringSplitOptions.RemoveEmptyEntries))
+        {
+            string language = segment.Split(';', 2)[0].Trim();
+            if (TryNormalizeLanguage(language, out bool isJapanese))
+                return isJapanese;
+        }
+
+        return false;
+    }
 }