In fact navi (and watashi, I guess you already know) can speak Japanese.

For me, the only knowledge I have about Japanese is how to read Hiragana and Katakana. I can't speak Japanese but I can understand the problem by following elements:

1. Chinese characters in Japanese.
2. Math notations and variable names in English.
3. Example Input/Output and Figures.
4. Some loan word composed by Katakana. I can guess the meaning by my English knowledge.

But sometimes it's very hard for me to understand the problem. For example, I spend more than 1 hour to figure out the meaning of this problem during the contest: http://kupc2012.contest.atcoder.jp/tasks/kupc2012_6

Tasks wrote by Japanese are very interesting, so I often participate in contests at AtCoder.