Implement cross-platform system UI language detection

Author: SHIINASAMA

app/__main__.py

@@ -6,6 +6,7 @@
 from qasync import QApplication, run
 
 from app.builtin.gitlab_updater import GitlabUpdater
+from app.builtin.locale import detect_system_ui_language
 from app.main_window import MainWindow
 from qdarktheme import enable_hi_dpi
 
@@ -48,7 +49,7 @@ def main(enable_updater: bool = True):
 
     # i18n
     translator = QTranslator()
-    lang_code = QLocale.system().name()
+    lang_code = detect_system_ui_language()
     translator.load(f":/i18n/{lang_code}.qm")
     app.installTranslator(translator)
 

app/builtin/locale.py

@@ -0,0 +1,19 @@
+def detect_system_ui_language():
+    import platform
+    system = platform.system()
+
+    if system == "Windows":
+        import ctypes, locale
+        lang_id = ctypes.windll.kernel32.GetUserDefaultUILanguage()
+        return locale.windows_locale.get(lang_id)
+
+    elif system == "Darwin":  # macOS
+        from Foundation import NSUserDefaults
+        langs = NSUserDefaults.standardUserDefaults().objectForKey_("AppleLanguages")
+        return langs[0] if langs else None
+    
+    else:  # Linux
+        import os
+        return (os.environ.get("LANG") or
+                os.environ.get("LC_ALL") or
+                os.environ.get("LC_MESSAGES"))

pyproject.toml

@@ -10,14 +10,15 @@ dependencies = [
     "qasync>=0.28.0",
     "glom>=24.11.0",
     "pyside6>=6.1.3",
+    "pyqtdarktheme-fork>=2.3.4",
     "singleton-decorator>=1.0.0",
 ]
 
 [dependency-groups]
 dev = [
     "nuitka>=2.8.1",
     "pyinstaller>=6.16.0",
-    "pyside-cli>=0.3.1",
+    "pyside-cli>=0.3.2",
     "pytest>=8.3.5",
     "pytest-benchmark>=4.0.0",
     "pytest-cov>=5.0.0",