Hmm. This is the translation page I used:
http://translate.google.com/?hl=en#
The result of the first English->Japanese:
サルが私のお尻のグレンベックは賢明な考えを口に知っているでしょうから飛び立つ。
The result of the reverse:
Gronbech the bottom of the monkeys will fly out of my you know the one sensible idea.
And it goes on from there.
Perhaps you just hit "swap" without copying and pasting the Japanese text into the text box? The "swap" button simply swaps the language direction, it doesn't change which block of text it takes as input, you have to manually do that.
ETA: Never mind, you figured it out.