Add wxTranslations::GetBestTranslation().

Implement preferred language selection on modern systems (OS X, Windows
Vista+). User settings for locale (aka "regional settings") and UI
language are independent there and the UI language shouldn't be
determined from the locale.

Moreover, the OS provides a list of preferred languages, not a single
value (as with locale), so we should use the best language given user's
preferences and available translations. A Czech user may prefer Slovak
UI over English, for example, and we should use Slovak translation in
absence of Czech one in that case instead of falling back to English.

On Unix, locale is language and so things remain as before.

Notice that calling wxLocale::Init(wxLANGUAGE_DEFAULT) does the right
thing now: it sets the locale to whatever the user has configured in
regional settings and loads translations corresponding to default
wxTranslations language, which is determined as described above.
Previously, UI would be translated using a language corresponding to the
regional settings.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72430 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/intl.h

@@ -391,10 +391,17 @@ public:
     static wxString GetSystemEncodingName();
 
     /**
-        Tries to detect the user's default language setting.
+        Tries to detect the user's default locale setting.
 
         Returns the ::wxLanguage value or @c wxLANGUAGE_UNKNOWN if the language-guessing
         algorithm failed.
+
+        @note This function works with @em locales and returns the user's default
+              locale. This may be, and usually is, the same as their preferred UI
+              language, but it's not the same thing. Use wxTranslation to obtain
+              @em language information.
+
+        @see wxTranslations::GetBestTranslation().
     */
     static int GetSystemLanguage();