Improve locale handling

- fgfs --language=fr-FR now works as expected for the launcher (also with
  --language=fr_FR as before): the value added by option --language to
  the _languages member of FGLocale is normalized in "underscore" style
  (e.g., fr_FR).

- Add-on translations must now use a hyphen in their property node names
  in addon-metadata.xml (talking about children of <localized>): i.e.,
  use for instance <fr-FR> there, not <fr_FR>. An exception
  (addons::errors::error_loading_metadata_file) is thrown if one of
  these nodes has a name containing an underscore.

src/Add-ons/AddonMetadataParser.cxx

@@ -73,6 +73,29 @@ static string getMaybeLocalized(const string& tag, SGPropertyNode* base, SGPrope
     return {};
 }
 
+static SGPropertyNode* getAndCheckLocalizedNode(SGPropertyNode* addonNode,
+                                                const SGPath& metadataFile)
+{
+    const auto localizedNode = addonNode->getChild("localized");
+    if (!localizedNode) {
+        return nullptr;
+    }
+
+    for (int i = 0; i < localizedNode->nChildren(); ++i) {
+        const auto node = localizedNode->getChild(i);
+        const string& name = node->getNameString();
+
+        if (name.find('_') != string::npos) {
+            throw errors::error_loading_metadata_file(
+                "underscores not allowed in names of children of <localized> "
+                "(in add-on metadata file '" + metadataFile.utf8Str() + "'); "
+                "hyphens should be used, as in 'fr-FR' or 'en-GB'");
+        }
+    }
+
+    return localizedNode;
+}
+
 // Static method
 Addon::Metadata
 Addon::MetadataParser::parseMetadataFile(const SGPath& addonPath)
@@ -141,7 +164,7 @@ Addon::MetadataParser::parseMetadataFile(const SGPath& addonPath)
       metadataFile.utf8Str() + "'");
   }
 
-  SGPropertyNode* localizedNode = addonNode->getChild("localized");
+  const auto localizedNode = getAndCheckLocalizedNode(addonNode, metadataFile);
   SGPropertyNode* langStringsNode = globals->get_locale()->selectLanguageNode(localizedNode);
 
   SGPropertyNode *idNode = addonNode->getChild("identifier");

src/GUI/QtLauncher.cxx

@@ -46,6 +46,7 @@
 #include <simgear/structure/exception.hxx>
 #include <simgear/structure/subsystem_mgr.hxx>
 #include <simgear/misc/sg_path.hxx>
+#include <simgear/misc/strutils.hxx>
 #include <simgear/package/Root.hxx>
 #include <simgear/package/Catalog.hxx>
 #include <simgear/package/Package.hxx>
@@ -292,7 +293,10 @@ void initApp(int& argc, char** argv, bool doInitQSettings)
         QTranslator* translator = new QTranslator(static_qApp.get());
         // check for --langauge=xx option and prefer that over QLocale
         // detection of the locale if it exists
-        auto lang = Options::getArgValue(argc, argv, "--language");
+        auto lang = simgear::strutils::replace(
+            Options::getArgValue(argc, argv, "--language"),
+            "-",
+            "_");
         if (!lang.empty()) {
             QString localeFile = "FlightGear_" + QString::fromStdString(lang);
             if (translator->load(localeFile, QLatin1String(":/"))) {

src/Main/locale.cxx

@@ -29,6 +29,7 @@
 #include <cstddef>              // std::size_t
 #include <cassert>
 
+#include <simgear/misc/strutils.hxx>
 #include <simgear/props/props.hxx>
 #include <simgear/props/props_io.hxx>
 #include <simgear/structure/exception.hxx>
@@ -41,6 +42,7 @@
 
 using std::vector;
 using std::string;
+namespace strutils = simgear::strutils;
 
 FGLocale::FGLocale(SGPropertyNode* root) :
 	_intl(root->getNode("/sim/intl", 0, true)),
@@ -178,6 +180,7 @@ FGLocale::findLocaleNode(const string& localeSpec)
        }
     }
 
+    // try country's default resource, i.e. "de_DE" => "de"
     const auto justTheLanguage = removeLocalePart(language);
     if (!justTheLanguage.empty()) {
         node = findLocaleNode(justTheLanguage);
@@ -201,7 +204,8 @@ bool FGLocale::selectLanguage(const std::string& language)
 
     // if we were passed a language option, try it first
     if (!language.empty()) {
-        _languages.insert(_languages.begin(), language);
+        const auto normalizedLang = strutils::replace(language, "-", "_");
+        _languages.insert(_languages.begin(), normalizedLang);
     }
 
     _currentLocaleString = removeEncodingPart(_languages.front());
@@ -600,7 +604,10 @@ SGPropertyNode_ptr FGLocale::selectLanguageNode(SGPropertyNode* langs) const
         return {};
 
     for (auto l : _languages) {
-        const auto langNoEncoding = removeEncodingPart(l);
+        // Only accept the hyphen separator in PropertyList node names between
+        // language and territory
+        const auto langNoEncoding = strutils::replace(removeEncodingPart(l),
+                                                      "_", "-");
         if (langs->hasChild(langNoEncoding)) {
             return langs->getChild(langNoEncoding);
         }