Your language-en.js is overwriting langHeatWords as defined in ajaxWDwx3.js, shortening the array. So the index 14 which applies to the lowest windchill setting (see function ajax_getHeatColor() in the latter file) is out of bounds; hence the undefined display.
Changing language-en.js to match the array definition in ajaxWDwx3.js should fix it. You'll need to reload javascript (usually SHIFT-refresh in the browser) to load the new code.