Thanks for the reply.
I commented out the two offending lines of code:
// if (WPLANG != ”)
// setlocale(LC_ALL, WPLANG.’.UTF-8′);
and all is now good with the child theme.
There doesn’t seem to be a problem with the plug-in so I/m thinking that this is just some left-over code from earlier versions. The strange thing is that there was no problem with it under WP 4.5.7; It jumped up when I upgraded to 4.6.
Thanks again for your help.