fix: use the correct font family for CJK characters

src/Ryujinx/UI/Applet/AvaloniaHostUITheme.cs

@@ -2,14 +2,29 @@ using Avalonia.Media;
 using Ryujinx.Ava.UI.Windows;
 using Ryujinx.HLE.UI;
 using System;
+using System.Globalization;
 
 namespace Ryujinx.Ava.UI.Applet
 {
     class AvaloniaHostUITheme : IHostUITheme
     {
+        private readonly MainWindow _parent;
+
+        public string FontFamily { get; }
+        public ThemeColor DefaultBackgroundColor { get; }
+        public ThemeColor DefaultForegroundColor { get; }
+        public ThemeColor DefaultBorderColor { get; }
+        public ThemeColor SelectionBackgroundColor { get; }
+        public ThemeColor SelectionForegroundColor { get; }
+
         public AvaloniaHostUITheme(MainWindow parent)
         {
-            FontFamily = OperatingSystem.IsWindows() && OperatingSystem.IsWindowsVersionAtLeast(10, 0, 22000) ? "Segoe UI Variable" : parent.FontFamily.Name;
+            _parent = parent;
+
+            // Initialize font property
+            FontFamily = GetSystemFontFamily();
+
+            // Initialize all properties that depend on parent
             DefaultBackgroundColor = BrushToThemeColor(parent.Background);
             DefaultForegroundColor = BrushToThemeColor(parent.Foreground);
             DefaultBorderColor = BrushToThemeColor(parent.BorderBrush);
@@ -17,24 +32,74 @@ namespace Ryujinx.Ava.UI.Applet
             SelectionForegroundColor = BrushToThemeColor(parent.ViewControls.SearchBox.SelectionForegroundBrush);
         }
 
-        public string FontFamily { get; }
+        private string GetSystemFontFamily()
+        {
+            if (OperatingSystem.IsWindows())
+            {
+                return GetWindowsFontByLanguage();
+            }
+            else if (OperatingSystem.IsMacOS())
+            {
+                return GetMacOSFontByLanguage();
+            }
+            else // Linux and other platforms
+            {
+                return GetLinuxFontByLanguage();
+            }
+        }
 
-        public ThemeColor DefaultBackgroundColor { get; }
+        private string GetWindowsFontByLanguage()
-        public ThemeColor DefaultForegroundColor { get; }
+        {
-        public ThemeColor DefaultBorderColor { get; }
+            var culture = CultureInfo.CurrentUICulture;
-        public ThemeColor SelectionBackgroundColor { get; }
+            string langCode = culture.Name;
-        public ThemeColor SelectionForegroundColor { get; }
+
+            return culture.TwoLetterISOLanguageName switch
+            {
+                "zh" => langCode == "zh-CN" || langCode == "zh-Hans" || langCode == "zh-SG"
+                    ? "Microsoft YaHei UI"  // Simplified Chinese
+                    : "Microsoft JhengHei UI", // Traditional Chinese
+
+                "ja" => "Yu Gothic UI",     // Japanese
+                "ko" => "Malgun Gothic",     // Korean
+                _ => OperatingSystem.IsWindowsVersionAtLeast(10, 0, 22000)
+                    ? "Segoe UI Variable"   // Other languages - Windows 11+
+                    : _parent.FontFamily.Name // Fallback to parent window font
+            };
+        }
+
+        private string GetMacOSFontByLanguage()
+        {
+            return CultureInfo.CurrentUICulture.TwoLetterISOLanguageName switch
+            {
+                "zh" => "PingFang SC",      // Chinese (both simplified and traditional)
+                "ja" => "Hiragino Sans",    // Japanese
+                "ko" => "Apple SD Gothic Neo", // Korean
+                _ => _parent.FontFamily.Name // Fallback to parent window font
+            };
+        }
+
+        private string GetLinuxFontByLanguage()
+        {
+            return CultureInfo.CurrentUICulture.TwoLetterISOLanguageName switch
+            {
+                "zh" => "Noto Sans CJK SC", // Chinese
+                "ja" => "Noto Sans CJK JP", // Japanese
+                "ko" => "Noto Sans CJK KR", // Korean
+                _ => _parent.FontFamily.Name // Fallback to parent window font
+            };
+        }
 
         private static ThemeColor BrushToThemeColor(IBrush brush)
         {
             if (brush is SolidColorBrush solidColor)
             {
-                return new ThemeColor((float)solidColor.Color.A / 255,
+                return new ThemeColor(
+                    (float)solidColor.Color.A / 255,
                     (float)solidColor.Color.R / 255,
                     (float)solidColor.Color.G / 255,
-                    (float)solidColor.Color.B / 255);
+                    (float)solidColor.Color.B / 255
+                );
             }
-
             return new ThemeColor();
         }
     }