OK, I admit it right away - I understood almost no dialogue in Avvai Shanmugi. I watched it in the theatre in India with some friends. However, Kamala Hassan is a great actor and the plot seemed evident enough (and my friends helped).
I had not seen the American Mrs. Doubtfire that this is based on, so I cannot make the obvious comparison and contrast. However, Kamala Hassan made this movie a treat to watch. The movie progressed a little slowly, but most every scene was funny. The plot is relatively evident, even a non-Tamil speaker will enjoy it.
If you get an opportunity to see this movie, I would give it a try. If you have already seen Mrs. Doubtfire, you might be interested to see how the Tamil movie industry dealt with this theme.