I have this project for school with my students and we are trying to create a list of the non-native accents in English that are the most difficult to understand (by natives or non natives). In other words, I'm trying to establish if it is English spoken in France, Poland, or Liberia etc. Also, what are the deviant phonological features that make comprehension more difficult? (other than word stress on the wrong syllable). If anyone has thoughts or knows a study regarding these topics, I would be very glad to hear about it. Thanks for your help!