Defensive handling of locales.

- Use setlocale() to ensure consistent handling of locales and
  string handling irrespective of whether or not QCoreApplication
  is invoked. Forces a the C locale for numerics and collation,
  since many pieces of FG assume this.

src/GUI/QtLauncher.cxx

@@ -21,6 +21,8 @@
 #include "QtLauncher.hxx"
 #include "QtLauncher_private.hxx"
 
+#include <locale.h>
+
 // Qt
 #include <QProgressDialog>
 #include <QCoreApplication>
@@ -332,14 +334,15 @@ namespace flightgear
 
 void initApp(int& argc, char** argv)
 {
-    sglog().setLogLevels( SG_ALL, SG_INFO );
-    initQtResources(); // can't be called from a namespace
-
     static bool qtInitDone = false;
     static int s_argc;
 
     if (!qtInitDone) {
         qtInitDone = true;
+
+        sglog().setLogLevels( SG_ALL, SG_INFO );
+        initQtResources(); // can't be called from a namespace
+
         s_argc = argc; // QApplication only stores a reference to argc,
         // and may crash if it is freed
         // http://doc.qt.io/qt-5/qguiapplication.html#QGuiApplication
@@ -349,6 +352,11 @@ void initApp(int& argc, char** argv)
         app->setApplicationName("FlightGear");
         app->setOrganizationDomain("flightgear.org");
 
+        // reset numeric / collation locales as described at:
+        // http://doc.qt.io/qt-5/qcoreapplication.html#details
+        ::setlocale(LC_NUMERIC, "C");
+        ::setlocale(LC_COLLATE, "C");
+
         // avoid double Apple menu and other weirdness if both Qt and OSG
         // try to initialise various Cocoa structures.
         flightgear::WindowBuilder::setPoseAsStandaloneApp(false);

src/Main/bootstrap.cxx

@@ -50,6 +50,7 @@
 #include <stdio.h>
 #include <cstring>
 #include <iostream>
+#include <locale.h>
 
 #include <simgear/compiler.h>
 #include <simgear/structure/exception.hxx>
@@ -213,6 +214,13 @@ int main ( int argc, char **argv )
 
     initFPE(flightgear::Options::checkForArg(argc, argv, "enable-fpe"));
 
+    // pick up all user locale settings, but force C locale for numerical/sorting
+    // conversions because we have lots of code which assumes standard
+    // formatting
+    setlocale(LC_ALL, "");
+    setlocale(LC_NUMERIC, "C");
+    setlocale(LC_COLLATE, "C");
+
     bool fgviewer = flightgear::Options::checkForArg(argc, argv, "fgviewer");
     try {
         // http://code.google.com/p/flightgear-bugs/issues/detail?id=1231