I don't know about that festival, but there are festivals all over Belgium all year long. See here (2012 is just starting to get filled in):https://docs.google.com/spreadsheet/ccc?key=0AlzngWrA6XiTdFlZRGw0YTdPaWFfWkIxTmxqNWkxWlE#gid=1
Zythos (ZBF) in April is the biggest. My personal favorite is the Hasselt Weekend of Beer happening mid-November.
No, you shouldn't have a problem with language, as long as you're not randomly striking up conversation with passers-by. It's not true that "nearly everyone speaks English", but enough know enough that you'll have no problem getting by. English is far more widespread among the younger generation. I have my own rule: Under 30, go straight to English; over 30, give Flemish a try.