Allow setting UI language from commandline (implements #678)

src/init.cpp

@@ -197,6 +197,9 @@ bool AppInit2(int argc, char* argv[])
             "  -connect=<ip>    \t\t  " + _("Connect only to the specified node") + "\n" +
             "  -irc             \t  "   + _("Find peers using internet relay chat (default: 0)") + "\n" +
             "  -listen          \t  "   + _("Accept connections from outside (default: 1)") + "\n" +
+#ifdef QT_GUI
+            "  -lang=<lang>     \t\t  " + _("Set language, for example \"de_DE\" (default: system locale)") + "\n" +
+#endif
             "  -dnsseed         \t  "   + _("Find peers using DNS lookup (default: 1)") + "\n" +
             "  -banscore=<n>    \t  "   + _("Threshold for disconnecting misbehaving peers (default: 100)") + "\n" +
             "  -bantime=<n>     \t  "   + _("Number of seconds to keep misbehaving peers from reconnecting (default: 86400)") + "\n" +

src/qt/bitcoin.cpp

@@ -162,11 +162,13 @@ int main(int argc, char *argv[])
 
     ParseParameters(argc, argv);
 
-    // Load language files for system locale:
+    // Get desired locale ("en_US") from command line or system locale
+    QString lang_territory = QString::fromStdString(GetArg("-lang", QLocale::system().name().toStdString()));
+    // Load language files for configured locale:
     // - First load the translator for the base language, without territory
     // - Then load the more specific locale translator
-    QString lang_territory = QLocale::system().name(); // "en_US"
     QString lang = lang_territory;
+
     lang.truncate(lang_territory.lastIndexOf('_')); // "en"
     QTranslator qtTranslatorBase, qtTranslator, translatorBase, translator;