It seems to work OK, but that looks like an error message. Is it anything to worry about? Is there any easy way of avoiding it?
If it appears to work, then I shouldn't worry too much about it. You can try adding a "lang=" attribute to the xsl:sort command, so the XSLT processor won't worry about what you mean (see http://www.w3schools.com/xsl/el_sort.asp). I think the message is to remind you that there might be a difference between international and 'native' (system?) sorting.
The lang= attribute accepts international language abbrevs such as "en", "de", "fr", "en-GB", and "en-US".
That is an eminently sensible suggestion, and it never occurred to me, but that doesn't seem to be it. All the same, it isn't really a problem -- it just gives the operation of the script a slightly clunky feel!