I did not find any option that can modify language in the security container.
Hope mcafee improve this issue.
Secure container defaults to the OS language setting. if you want to enable Japanese, you can change your device language to Japanese and the secure container will also switch its locale to Japanese.
additionally, a newer version of secure container is due out late this month that will contain support for additional languages.
You are right, it depends on the OS language,But still there are many bugs in the user interface of secure container.
see below caputered picture for e.g. The Android OS is set to Chinese, but the container shows Chinese and English Mixed.
and some place:wrting email,address book... are only English. Hope next verstion can fix it. Thanks.