It looks like your hosting has disabled the PHP putenv() function. If you have access to your own server setup this may be something you can re-enable. Otherwise, to resolve the error you may need to comment out those lines (59-61 in src/Gibbon/Locale.php, 26 in index_tt_ajax.php). In this case it’s a PHP warning rather than an error, so it’s not fatal to silence the error by commenting these lines out, as the setlocale() function after it should be sufficient.
I don’t think we’ve seen this issue before, so it is perhaps something we can add a check for in future versions to see if the server supports putenv() before calling it.