Set C locale for wxLANGUAGE_DEFAULT under Unix too

This had been done before all the recent wxUILocale-related changed and
was still done for non-Unix systems, but not there because we explicitly
skipped calling setlocale() in this case with a comment saying that the
default locale was already in use -- however this comment was wrong and
we do need to call setlocale(LC_ALL, "") in this case for compatibility
and consistency.

Closes #22212.

src/unix/uilocale.cpp

@@ -258,12 +258,9 @@ wxUILocaleImplUnix::~wxUILocaleImplUnix()
 void
 wxUILocaleImplUnix::Use()
 {
-    if ( m_locId.IsEmpty() )
-    {
-        // This is the default locale, it is already in use.
-        return;
-    }
-
+    // Note that we don't need to test if m_locId is not empty here, as we
+    // still want to call setlocale() with the empty string to use the default
+    // locale settings in this case.
     if ( !wxSetlocaleTryAll(LC_ALL, m_locId) )
     {
         // Some C libraries (namely glibc) still use old ISO 639,