this I found in my library:
"Windows: go to the ADE installed folder ( For e.g: C:\Program Files\Adobe\Adobe Digital Editions 2.0\) rename the current language folder (For e.g C:\Program Files\Adobe\Adobe Digital Editions 2.0\fr_). If ADE could not find the OS language setting folder, then it will automatically switch to English menus. or if you dont want other languages, just delete those language folders."
I solved this one myself by finding out where Firefox keeps its cookies and deleting the ones set by adobe.