Because, frankly, none of us come to law school knowing the Japanese terminology for tort, negligence, or statute of limitations. (不法行為、 過失、時効、 respectively).
Japanese Law Translation has on online Japanese-English dictionary, as well as English translations of current Japanese law available onilne, for free, at its site. The dictionary is quite good, though there may be some terms lacking (I was unable to get it to produce “Alternative Dispute Resolution,” or 裁判外紛争解決, for example). Nonetheless, it can be a good place to access some Japanese legal terminology. And, at the very least the site can equip you with the kanji compounds for quick shorthand note-taking in the margins of your casebook.
The site is located here.