Cloe,
Application requirements may differ depending on the year, but currently for example, for new college graduates, the requirements for applying to be flight attendants on JAL are as follows;
1) Those due to graduate an university or junior college during April 2008 to March 2009.
2) Those with English language ability equavalent to TOEIC 600 points or more.
3) Those with no health problems in respiratory or circulatory organs, no ear/nose/throat/eye/spine disorders, and corrected eyesight of 1.0 or better (contact lense use).
4) Those living or has plans to live within 40km from Haneda Airport, and is able to commute to the Airport within 90 minutes using public transportation.
5) Those who can work on early morning shifts, midnight shifts and is able to stay overnight, regardless or the date.
6) Those able to be employed any designated date after April 2009.
7) Nationality is not an issue.
However, on the requirements for those who already have graduated, it doesn't mention anything about the nationality.
Keep in mind that if you are granted to become an employee after competitive tests, you will be trained for a couple of months, then you will be flying domestic flights for a couple of years, and finally you will be sent to international flights.
This information all comes from the Japanese version of JAL's official website.
http://www.jal.com/ja/saiyo/apply/cabin_kisotsu.html