I don't know that there's one best, it really depends on which country you'd like to interact with and possibly live in the most.
As a general rule I've been told that foreign languages are only useful if you are sufficiently good in them to be able to conduct an entire interview and write a legal brief in the language. This is probably an exaggeration, but is what the career services office said at my school.
Note that there's not really a single, unified field of international law. That could include anything from international M&A to human rights to war crimes prosecutions. Japanese probably won't help much for the latter two, it could be quite helpful if you wanna do US-Japanese deals/commercial law.