Problems with communication:
– too many different incompatible languages, especially radically different ones like Arabic vs English vs Mandarin
– significantly large populations speaking incompatible languages.
– Print, Phone, Letters, Mobile phones, Computers – too many different devices, some of which are incompatible with each other and possibly also run different frequencies and codes.
– Networks – different companies, different standards, different chips, different antennas. ugh
– Programs – IRC, HTTP, Snapchat, fb messenger, Google Hangouts, Naver Line, Facebook WhatsApp, Hipchat, Slack, WeChat, etc etc etc etc etc. Way too many to list. All with their own respective problems.
tl;dr the aliens trying to invade Earth are going to have a hard time.