Module doesn't work on non-English single language website

I found that the variable $entity->language results to the language code 'de' for a piece of node content on a German single language website. When this value is being used as an index for an array in several lines of code - e.g. "empty($entity->{$field_name}[$entity->language])" in the file "references_manager.pages.inc" - the program breaks, because the array item would need the index value "und" (=undefined). I've not further investigated, but this would have to be solved in at least 3 places to get the programm running on such a foreign website.