Launcher respects --language arg if set

src/GUI/QtLauncher.cxx

@@ -281,7 +281,7 @@ void initApp(int& argc, char** argv, bool doInitQSettings)
           QStringLiteral("org.flightgear.FlightGear.desktop"));
 #endif
         QTranslator* fallbackTranslator = new QTranslator(static_qApp.get());
-        if (!fallbackTranslator->load(QLatin1String(":/FlightGear_en_US.qm"))) {
+        if (!fallbackTranslator->load(QLatin1String(":/FlightGear_en.qm"))) {
             qWarning() << "Failed to load default (en) translations";
             delete fallbackTranslator;
         } else {
@@ -289,7 +289,19 @@ void initApp(int& argc, char** argv, bool doInitQSettings)
         }
 
         QTranslator* translator = new QTranslator(static_qApp.get());
-        if (translator->load(QLocale(), QLatin1String("FlightGear"), QLatin1String("_"), QLatin1String(":/"))) {
+        // check for --langauge=xx option and prefer that over QLocale
+        // detection of the locale if it exists
+        auto lang = Options::getArgValue(argc, argv, "--language");
+        if (!lang.empty()) {
+            QString localeFile = "FlightGear_" + QString::fromStdString(lang);
+            if (translator->load(localeFile, QLatin1String(":/"))) {
+                qInfo() << "Loaded translations based on --language from:" << localeFile;
+                static_qApp->installTranslator(translator);
+            } else {
+                qInfo() << "--langauge was set, but no translations found at:" << localeFile;
+                delete translator;
+            }
+        } else if (translator->load(QLocale(), QLatin1String("FlightGear"), QLatin1String("_"), QLatin1String(":/"))) {
             // QLocale().name() looks like ' "it_IT" ' (without the outer
             // quotes) when running FG on Linux with LANG=it_IT.UTF-8.
             qInfo() << "Loaded translations for locale" << QLocale().name();

src/Main/options.cxx

@@ -251,7 +251,7 @@ void fgSetDefaults ()
     v->setValueReadOnly("build-type", FG_BUILD_TYPE);
 
     char* envp = ::getenv( "http_proxy" );
-    if( envp != NULL )
+    if( envp != nullptr )
       fgSetupProxy( envp );
 }
 
@@ -2960,7 +2960,7 @@ bool Options::checkForArg(int argc, char* argv[], const char* checkArg)
 {
     for (int i = 0; i < argc; ++i) {
         char* arg = argv[i];
-        if (arg == 0) {
+        if (arg == nullptr) {
             continue;
         }
 
@@ -2981,4 +2981,24 @@ bool Options::checkForArg(int argc, char* argv[], const char* checkArg)
     return false;
 }
 
+std::string Options::getArgValue(int argc, char* argv[], const char* checkArg)
+{
+    const auto len = strlen(checkArg);
+    for (int i = 0; i < argc; ++i) {
+        char* arg = argv[i];
+        if (arg == nullptr) {
+            continue;
+        }
+
+        if (strncmp(arg, checkArg, len) == 0) {
+            const auto alen = strlen(arg);
+            if ((alen - len) < 2)
+                return {}; // no value after the =, or missing = entirely
+            return std::string(arg + len + 1);
+        }
+    } // of args iteration
+
+    return {};
+}
+
 } // of namespace flightgear

src/Main/options.hxx

@@ -155,6 +155,17 @@ public:
    */
   static bool checkForArg(int argc, char* argv[], const char* arg);
 
+  /**
+   * @brief getArgValue - get the value of an argument if it exists, or
+   * an empty string otherwise
+   * @param argc
+   * @param argv
+   * @param checkArg : arg to look for, with '--' prefix
+   * @return value following '=' until the next white space
+   */
+  static std::string getArgValue(int argc, char* argv[], const char* checkArg);
+
+
       SGPath platformDefaultRoot() const;
 
       /**