Films of plays have to overcome lots of problems of scale. Audiences naturally want movies to supply things they can't get from television, and American television, in particular, supplies some very well-written and acted drama on a nightly basis.
Tom, I haven't seen it for years, but I never realised The Never Ending Story was German. I wouldn't be surprised if that is the most universally popular German film then? It was a huge hit and I remember seeing it as a kid. I can't think of any others.
My top 5 Canadian films, in no particular order....Crash (Cronenberg version)The Decline Of The American EmpirePasschendaeleBlack Robe VideodromeI could have easily made this a Top 10.