The purpose of this study was to apply two types of Differential Item Functioning (DIF), net and global DIF, as well as the framework of Differential Step Functioning (DSF) to real testing data to investigate measurement invariance related to test language. Data from the Program for International Student Assessment (PISA)-2006 polytomously scored science items for four countries with different test languages were used, where French and English represented the reference languages. The findings showed that many items exhibited both types of DIF, although, in most cases, the results were inconsistent for the two source languages. In addition, net and global DIF tests did not always yield the same results depending on the DSF effect pattern. Furthermore, the DSF analysis provided valuable information over and above that provided by the net DIF analysis concerning the nature and the location of the DIF effect.