Yes. I love those descriptive errormessages. However the problem is that i can not imagine how this can ever happen.
It would mean that
_wppaAvg[mocc][_wppaCurIdx[mocc]] is also undefiend etc...
Higher up in the code it reads ( where the array is filled with data ):
_wppaAvg[mocc][id] = avgrat;
avgrat is an argument of
wppaStoreSlideInfo(), that is filled by the php routien
wppa_get_slide_info(), which is performed properly otherwise you would not have the slide anyway. See this line from your pages source:
wppaStoreSlideInfo('1','0','/7.jpg?ver=1',' max-width:640px; max-height:480px; width:640px; height:480px;margin:0;','640','480','Origami de Renée CASTAGNO','Origami de Renée CASTAGNO','Représentation du logo Onet en 3D\nCette œuvre est réalisée afin de mettre en avant le nouveau logo d\'Onet.','7','|0','0','0','http://toutunmondedetalents.groupeonet.com/autres-arts-plastiques/?wppa-album=5&wppa-cover=0&wppa-slide&wppa-occur=1&wppa-photo=7','','','','0','','','','','http://toutunmondedetalents.groupeonet.com/autres-arts-plastiques/?wppa-album=5&wppa-photo=7&wppa-cover=0&wppa-occur=1','');
where the argument
'|0' is avgrat.
'|0' is splittable into an empty string and value 0, i.e. NOT undefined...
I could fix avg itsself by testing if it is defined, but that is not the cause of the problem, and what would the content be then?
There must be some mechanism that interferes somehow with these js functions. The exact situation ( except for your extra plugins ) runs without problems on my site, even with a one image slideshow, like yours: http://beta.opajaap.nl/generic/?lang=en&wppa-album=151&wppa-cover=0&wppa-occur=1&wppa-photo=1003320
A tip for troubleshooting:
You can delete wppa.min.js using ftp. It then loads wppa.js ( not minified ). This makes it easier to locate a js problem.
I just installed and activated polylang, a short test did not show it to be the problem.
Did you know that wppa+ supports qTranslate fully?
Temporary try de-activating all other plugins and/or test it on another theme to eliminate. If there might be an error in your download of wppa+, try deleting the plugin and re-install from the plugins page. You will never loose anything from wppa+ when removing the plugin.