The JPO announced, on March 25, 2016, that it will be possible to file foreign language applications in any language before the Japanese Patent Office.
For an application based on the Paris Convention, it is currently possible to file the application in English to secure a filing date. Foreign language applications in languages other than English are not permitted.
However, as of April 1, 2016, it will be possible to file the application in any language to secure a filing date.
Then, a Japanese translation must be filed within 14 months from the priority date.
It is not possible to enter the Japanese national phase of a PCT application in English or other foreign language.