Qt: Use selected language locale for formatting

But check the prefix against the system locale, so that e.g. en can use
the system country.

src/duckstation-qt/qthost.cpp

@@ -2383,8 +2383,7 @@ std::span<const std::pair<const char*, const char*>> Host::GetAvailableLanguageL
   static constexpr const std::pair<const char*, const char*> languages[] = {
     {QT_TRANSLATE_NOOP("QtHost", "System Language"), ""},
 
-#define TRANSLATION_LIST_ENTRY(name, our_translation_code, locale_code)                                                \
-  {name " [" our_translation_code "]", our_translation_code},
+#define TRANSLATION_LIST_ENTRY(name, code, locale_code, qlanguage, qcountry) {name " [" code "]", code},
 #include "qttranslations.inl"
 #undef TRANSLATION_LIST_ENTRY
   };
@@ -2488,19 +2487,30 @@ const QLocale& QtHost::GetApplicationLocale()
 
 void QtHost::UpdateApplicationLocale(std::string_view language)
 {
-#if 0
-  // Only for testing purposes. Keep the system locale for users.
-  if (language == "ja")
-    s_app_locale = QLocale(QLocale::Japanese, QLocale::Japan);
-  else if (language == "ko")
-    s_app_locale = QLocale(QLocale::Korean, QLocale::SouthKorea);
-  else if (language == "zh-CN")
-    s_app_locale = QLocale(QLocale::Chinese, QLocale::China);
-  else
-    s_app_locale = QLocale::system();
-#else
+  static constexpr const std::tuple<const char*, QLocale::Language, QLocale::Country> lookup[] = {
+#define TRANSLATION_LIST_ENTRY(name, code, locale_code, qlanguage, qcountry) {code, qlanguage, qcountry},
+#include "qttranslations.inl"
+#undef TRANSLATION_LIST_ENTRY
+  };
+
+  // If the system locale is using the same language, then use the system locale.
+  // Otherwise we'll be using that ugly US date format in Straya mate.
   s_app_locale = QLocale::system();
-#endif
+  const std::string system_locale_name = s_app_locale.name().toStdString();
+  if (s_app_locale.name().startsWith(QLatin1StringView(language), Qt::CaseInsensitive))
+  {
+    INFO_LOG("Using system locale for {}.", language);
+    return;
+  }
+
+  for (const auto& [code, qlanguage, qcountry] : lookup)
+  {
+    if (language == code)
+    {
+      s_app_locale = QLocale(qlanguage, qcountry);
+      return;
+    }
+  }
 }
 
 void Host::ReportDebuggerMessage(std::string_view message)

src/duckstation-qt/qttranslations.inl

@@ -1,20 +1,20 @@
 // SPDX-FileCopyrightText: 2019-2025 Connor McLaughlin <stenzek@gmail.com>
 // SPDX-License-Identifier: CC-BY-NC-ND-4.0
 
-// TRANSLATION_LIST_ENTRY(name, our_translation_code, locale_code)
+// TRANSLATION_LIST_ENTRY(name, code, locale_code, qlanguage, qcountry)
 
-TRANSLATION_LIST_ENTRY("English", "en", "en-US")
-TRANSLATION_LIST_ENTRY("Español de Latinoamérica", "es", "es-ES")
-TRANSLATION_LIST_ENTRY("Español de España", "es-ES", "es-ES")
-TRANSLATION_LIST_ENTRY("Français", "fr", "fr-FR")
-TRANSLATION_LIST_ENTRY("Bahasa Indonesia", "id", "id-ID")
-TRANSLATION_LIST_ENTRY("日本語", "ja", "ja-JA")
-TRANSLATION_LIST_ENTRY("한국어", "ko", "ko-KO")
-TRANSLATION_LIST_ENTRY("Italiano", "it", "it-IT")
-TRANSLATION_LIST_ENTRY("Polski", "pl", "pl-PL")
-TRANSLATION_LIST_ENTRY("Português (Pt)", "pt-PT", "pt-PT")
-TRANSLATION_LIST_ENTRY("Português (Br)", "pt-BR", "pt-BR")
-TRANSLATION_LIST_ENTRY("Русский", "ru", "ru-RU")
-TRANSLATION_LIST_ENTRY("Svenska", "sv", "sv-SV")
-TRANSLATION_LIST_ENTRY("Türkçe", "tr", "tr-TR")
-TRANSLATION_LIST_ENTRY("简体中文", "zh-CN", "zh-CN")
+TRANSLATION_LIST_ENTRY("English", "en", "en-US", QLocale::English, QLocale::UnitedStates)
+TRANSLATION_LIST_ENTRY("Español de Latinoamérica", "es", "es-ES", QLocale::Spanish, QLocale::LatinAmerica)
+TRANSLATION_LIST_ENTRY("Español de España", "es-ES", "es-ES", QLocale::Spanish, QLocale::Spain)
+TRANSLATION_LIST_ENTRY("Français", "fr", "fr-FR", QLocale::French, QLocale::France)
+TRANSLATION_LIST_ENTRY("Bahasa Indonesia", "id", "id-ID", QLocale::Indonesian, QLocale::Indonesia)
+TRANSLATION_LIST_ENTRY("日本語", "ja", "ja-JA", QLocale::Japanese, QLocale::Japan)
+TRANSLATION_LIST_ENTRY("한국어", "ko", "ko-KO", QLocale::Korean, QLocale::SouthKorea)
+TRANSLATION_LIST_ENTRY("Italiano", "it", "it-IT", QLocale::Italian, QLocale::Italy)
+TRANSLATION_LIST_ENTRY("Polski", "pl", "pl-PL", QLocale::Polish, QLocale::Poland)
+TRANSLATION_LIST_ENTRY("Português (Pt)", "pt-PT", "pt-PT", QLocale::Portuguese, QLocale::Portugal)
+TRANSLATION_LIST_ENTRY("Português (Br)", "pt-BR", "pt-BR", QLocale::Portuguese, QLocale::Brazil)
+TRANSLATION_LIST_ENTRY("Русский", "ru", "ru-RU", QLocale::Russian, QLocale::Russia)
+TRANSLATION_LIST_ENTRY("Svenska", "sv", "sv-SV", QLocale::Swedish, QLocale::Sweden)
+TRANSLATION_LIST_ENTRY("Türkçe", "tr", "tr-TR", QLocale::Turkish, QLocale::Turkey)
+TRANSLATION_LIST_ENTRY("简体中文", "zh-CN", "zh-CN", QLocale::Chinese, QLocale::China)