ve_locale_exists, does,

 old = strdup (setlocale (LC_ALL, NULL));
 if (setlocale (LC_MESSAGES, loc) != NULL)
   [...]
 setlocale (LC_MESSAGES, old);

But if the initial locale is mixed, then the results of setlocale (LC_ALL,
NULL) will be something like

 LC_CTYPE=en_US.UTF-8;LC_NUMERIC=en_US.UTF-8;LC_COLLATE=C

which doesn't work when passed to setlocale (LC_MESSAGES, old);
(though it would work if passed to setlocale (LC_ALL, old).
Result of this is that you get Walloon messages - the last
language in locale.alias.

So, the simple fix is to change the LC_ALL to LC_MESSAGES 

-  old = strdup (setlocale (LC_ALL, NULL));
+  old = strdup (setlocale (LC_MESSAGES, NULL));