Add method FGLocale::getPreferredLanguage()

This function returns the preferred "locale"[1] according to user choice
and/or settings (i.e., it is influenced by --language if passed,
otherwise by current locale/system settings). The return value never has
an encoding part. It is the empty string if nothing could be found,
otherwise should look like fr_BE or it_IT.

[1] "language" term used in the function name for consistency with the
    existing and related FGLocale::selectLanguage().

src/Main/locale.cxx

@@ -170,6 +170,10 @@ FGLocale::selectLanguage(const char *language)
         languages.insert(languages.begin(), string(language));
     }
 
+    _currentLocaleString = removeEncodingPart(languages[0]);
+    if (_currentLocaleString == "C") {
+        _currentLocaleString.clear();
+    }
 
     _currentLocale = NULL;
     BOOST_FOREACH(string lang, languages) {
@@ -199,6 +203,14 @@ FGLocale::selectLanguage(const char *language)
     return true;
 }
 
+// Return the preferred language according to user choice and/or settings
+// (e.g., 'fr_FR', or the empty string if nothing could be found).
+std::string
+FGLocale::getPreferredLanguage() const
+{
+    return _currentLocaleString;
+}
+
 // Load strings for requested resource and locale.
 // Result is stored below "strings" in the property tree of the given locale.
 bool

src/Main/locale.hxx

@@ -42,6 +42,18 @@ public:
      */
     bool        selectLanguage      (const char* language = NULL);
 
+    /** Return the preferred language according to user choice and/or settings.
+     *
+     *  Examples: 'fr_CA', 'de_DE'... or the empty string if nothing could be
+     *            found.
+     *
+     *  Note that this is not necessarily the same as the last value passed to
+     *  selectLanguage(), assuming it was non-null and non-empty, because the
+     *  latter may have an encoding specifier, while values returned by
+     *  getPreferredLanguage() never have that.
+     */
+    std::string getPreferredLanguage() const;
+
     /**
      * Load strings for requested resource, i.e. "menu", "options", "dialogs".
      * Loads data for current and default locale (the latter is the fallback).
@@ -123,6 +135,7 @@ protected:
     SGPropertyNode_ptr _intl;
     SGPropertyNode_ptr _currentLocale;
     SGPropertyNode_ptr _defaultLocale;
+    std::string _currentLocaleString;
 
 private:
     /** Return a new string with the character encoding part of the locale