Yes, Amos, that's the one! Thank you!

I found it at www.presidency.ucsb.edu/docs/pppus/nixon/1972/122.htm (sorry, I haven't learned to do the "blue clicky thing" yet.) Turns out that it was not quite as I remembered it. According to this source, after giving the French greeting the President said:

To all of you who have welcomed Mrs. Nixon and me so warmly on this occasion, I trust you will give me allowances for trying to speak in the language that I studied 37 years ago. When I tried it, the day before I came, on the top linguist in the American Government, General Walters, he said, "Go ahead. You speak French with a Canadian accent."

Not quite as insensitive as I remembered it, but still pretty embarassing, I think.

Thanks again!