Replace wxLocaleIdent ctor from language with FromTag()

This static function parses a subset of the language tags described in
BCP 47 (see https://www.rfc-editor.org/rfc/bcp/bcp47.txt).

Use the tag, as specified by this function, rather than the locale
identifiers components under MSW, where this should allow us to use even
locales that can't be described using just language-script-region.

src/common/uilocale.cpp

@@ -22,6 +22,8 @@
 
 #include "wx/uilocale.h"
 
+#include "wx/arrstr.h"
+
 #ifndef __WINDOWS__
     #include "wx/language.h"
 #endif
@@ -45,6 +47,96 @@ wxUILocale wxUILocale::ms_current;
 // wxLocaleIdent
 // ----------------------------------------------------------------------------
 
+/* static */
+wxLocaleIdent wxLocaleIdent::FromTag(const wxString& tag)
+{
+    // See section 2.01 of https://www.rfc-editor.org/rfc/bcp/bcp47.txt for the
+    // full syntax. Here we fully support just the subset we're interested in:
+    //
+    //  - Normal language tags (not private use or grandfathered ones).
+    //  - Only script and region, but not the extensions or extlangs.
+
+    // Language tags must always use ASCII.
+    if ( tag != tag.ToAscii() )
+        return wxLocaleIdent();
+
+    const wxArrayString& parts = wxSplit(tag, '-', '\0');
+    wxArrayString::const_iterator it = parts.begin();
+    if ( it == parts.end() )
+        return wxLocaleIdent();
+
+    // We have at least the language, so we'll return a valid object.
+    wxLocaleIdent locId;
+    locId.m_language = *it;
+
+    // Also store the full string, so that the platforms that support BCP 47
+    // natively can use it instead of reconstructing the string from our fields.
+    locId.m_tag = tag;
+
+    if ( ++it == parts.end() )
+        return locId;
+
+    // Advance to the next component we know about.
+    switch ( locId.m_language.length() )
+    {
+        case 2:
+        case 3:
+            // Looks like an ISO 639 code.
+            break;
+
+        default:
+            // It may be private use or grandfathered tag or just invalid
+            // syntax, but in any case we can't parse it further.
+            return locId;
+    }
+
+    // Skip extlangs that are 3 letters long, in contrast to 3 digit region
+    // codes.
+    while ( it->length() == 3 && !isdigit((*it)[0]) )
+    {
+        if ( ++it == parts.end() )
+            return locId;
+    }
+
+    switch ( it->length() )
+    {
+        case 2:
+        case 3:
+            // Either an ISO 3166-1 or UN M.49 region code.
+            locId.m_region = *it;
+            break;
+
+        case 4:
+            // Must be an ISO 15924 script.
+            locId.m_script = *it;
+            break;
+
+        default:
+            // This looks to be completely invalid.
+            return wxLocaleIdent();
+    }
+
+    // If we got the language and the region, we can't parse anything else
+    // (variants, extensions, private use) anyhow.
+    if ( !locId.m_region.empty() )
+        return locId;
+
+    // Otherwise we must have got the script above, so check if we have the
+    // region too.
+    if ( ++it == parts.end() )
+        return locId;
+
+    switch ( it->length() )
+    {
+        case 2:
+        case 3:
+            locId.m_region = *it;
+            break;
+    }
+
+    return locId;
+}
+
 wxLocaleIdent& wxLocaleIdent::Language(const wxString& language)
 {
     m_language = language;
@@ -75,6 +167,22 @@ wxLocaleIdent& wxLocaleIdent::Modifier(const wxString& modifier)
     return *this;
 }
 
+wxString wxLocaleIdent::GetTag() const
+{
+    if ( !m_tag.empty() )
+        return m_tag;
+
+    wxString tag = m_language;
+
+    if ( !m_script.empty() )
+        tag << '-' << m_script;
+
+    if ( !m_region.empty() )
+        tag << '-' << m_region;
+
+    return tag;
+}
+
 // ----------------------------------------------------------------------------
 // wxUILocale
 // ----------------------------------------------------------------------------