Fix remote image language priority to prefer English over no-language

Previously, images with no language were ranked higher (score 3) than
English images (score 2), causing poorly rated languageless images to
be selected over well-rated English alternatives for posters and logos.

Swap the priority so English is preferred over no-language images.
Backdrop images are unaffected as they have their own dedicated sorting.

Add unit tests for OrderByLanguageDescending.

Fixes #13310

2 files changed, 121 insertions, 4 deletions

diff --git a/MediaBrowser.Model/Extensions/EnumerableExtensions.cs b/MediaBrowser.Model/Extensions/EnumerableExtensions.cs
index 94f425229..7c9ee18ca 100644
--- a/MediaBrowser.Model/Extensions/EnumerableExtensions.cs
+++ b/MediaBrowser.Model/Extensions/EnumerableExtensions.cs
@@ -11,7 +11,7 @@ namespace MediaBrowser.Model.Extensions
     public static class EnumerableExtensions
     {
         /// <summary>
-        /// Orders <see cref="RemoteImageInfo"/> by requested language in descending order, prioritizing "en" over other non-matches.
+        /// Orders <see cref="RemoteImageInfo"/> by requested language in descending order, then "en", then no language, over other non-matches.
         /// </summary>
         /// <param name="remoteImageInfos">The remote image infos.</param>
         /// <param name="requestedLanguage">The requested language for the images.</param>
@@ -28,9 +28,9 @@ namespace MediaBrowser.Model.Extensions
                 {
                     // Image priority ordering:
                     //  - Images that match the requested language
-                    //  - Images with no language
                     //  - TODO: Images that match the original language
                     //  - Images in English
+                    //  - Images with no language
                     //  - Images that don't match the requested language
 
                     if (string.Equals(requestedLanguage, i.Language, StringComparison.OrdinalIgnoreCase))
@@ -38,12 +38,12 @@ namespace MediaBrowser.Model.Extensions
                         return 4;
                     }
 
-                    if (string.IsNullOrEmpty(i.Language))
+                    if (string.Equals(i.Language, "en", StringComparison.OrdinalIgnoreCase))
                     {
                         return 3;
                     }
 
-                    if (string.Equals(i.Language, "en", StringComparison.OrdinalIgnoreCase))
+                    if (string.IsNullOrEmpty(i.Language))
                     {
                         return 2;
                     }
diff --git a/tests/Jellyfin.Model.Tests/Extensions/EnumerableExtensionsTests.cs b/tests/Jellyfin.Model.Tests/Extensions/EnumerableExtensionsTests.cs
new file mode 100644
index 000000000..3b65a2636
--- /dev/null
+++ b/tests/Jellyfin.Model.Tests/Extensions/EnumerableExtensionsTests.cs
@@ -0,0 +1,117 @@
+using System.Linq;
+using MediaBrowser.Model.Extensions;
+using MediaBrowser.Model.Providers;
+using Xunit;
+
+namespace Jellyfin.Model.Tests.Extensions
+{
+    public class EnumerableExtensionsTests
+    {
+        [Fact]
+        public void OrderByLanguageDescending_PreferredLanguageFirst()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "en", CommunityRating = 5.0, VoteCount = 100 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 9.0, VoteCount = 200 },
+                new RemoteImageInfo { Language = null, CommunityRating = 7.0, VoteCount = 50 },
+                new RemoteImageInfo { Language = "fr", CommunityRating = 8.0, VoteCount = 150 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            Assert.Equal("de", result[0].Language);
+            Assert.Equal("en", result[1].Language);
+            Assert.Null(result[2].Language);
+            Assert.Equal("fr", result[3].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_EnglishBeforeNoLanguage()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = null, CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 3.0, VoteCount = 10 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            // English should come before no-language, even with lower rating
+            Assert.Equal("en", result[0].Language);
+            Assert.Null(result[1].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_SameLanguageSortedByRatingThenVoteCount()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "de", CommunityRating = 5.0, VoteCount = 100 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 9.0, VoteCount = 50 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 9.0, VoteCount = 200 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            Assert.Equal(200, result[0].VoteCount);
+            Assert.Equal(50, result[1].VoteCount);
+            Assert.Equal(100, result[2].VoteCount);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_NullRequestedLanguage_DefaultsToEnglish()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "fr", CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 5.0, VoteCount = 10 },
+            };
+
+            var result = images.OrderByLanguageDescending(null!).ToList();
+
+            // With null requested language, English becomes the preferred language (score 4)
+            Assert.Equal("en", result[0].Language);
+            Assert.Equal("fr", result[1].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_EnglishRequested_NoDoubleBoost()
+        {
+            // When requested language IS English, "en" gets score 4 (requested match),
+            // no-language gets score 2, others get score 0
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = null, CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 3.0, VoteCount = 10 },
+                new RemoteImageInfo { Language = "fr", CommunityRating = 8.0, VoteCount = 300 },
+            };
+
+            var result = images.OrderByLanguageDescending("en").ToList();
+
+            Assert.Equal("en", result[0].Language);
+            Assert.Null(result[1].Language);
+            Assert.Equal("fr", result[2].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_FullPriorityOrder()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "fr", CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = null, CommunityRating = 8.0, VoteCount = 400 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 7.0, VoteCount = 300 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 6.0, VoteCount = 200 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            // Expected order: de (requested) > en > no-language > fr (other)
+            Assert.Equal("de", result[0].Language);
+            Assert.Equal("en", result[1].Language);
+            Assert.Null(result[2].Language);
+            Assert.Equal("fr", result[3].Language);
+        }
+    }
+}