We just competed a similar project and had to consider many things. Three of which was managing content - sharing content & users across all sites, common assets (images, CSS etc), lastly, URLs.
Things to consider, content and users perspective:
1. We were okay with not translating ALL content. News releases were in English and just shared across all sites.
2. Users should have one login from the Japanese site to the main English site
1. Not all images required translations. Images of devices (iPhones etc...) did not have to be different
These had to be native - so site.com/company would be site.com/notre-groupe in french.
Because of our clearly defined needs we were able to make a decision - one install of a CMS (Expressionengine) that created multiple sites and could share content, users and certain variables. For instance if we had a number of clients we have, which would change every time a client was added - we could edit this number in one place and have it update on all sites.
Lastly - scalability. Will there soon be a time where you can hand off your Japanese site to an employee in Japan to manage content? Content workflow?
So I guess you need to define your needs. It will help you make a decision, choose a CMS or what not.
In our case, Expressionengine worked great. But WordPress Multisite and domain mapping can get you up and running rapidly as well.