That's not really how international commercial disputes happen. You generally sue in the most favorable jurisdiction where you have some presence/nexus (a US business always can use the US). The issue is that any judgment will then need to be taken to Japan to perform, although it's possible/likely MtGox has assets in the USA, so you wouldn't need to go to Japan. There also may be US/Japan treaties which are relevant.

I am not a lawyer, and international disputes are even more complex.

"7. The Agreement provides that the Defendants “irrevocably consent to the personal jurisdiction of and venue in the state and federal courts located in King County, Washington withrespect to any action, claim or proceeding arising out of or relating to this Agreement.”"