Japanese translation have reached the prerequisites from PEP 545, their last step is to be added to the language switcher.

Current build has been reviewed by Inada and is OK: https://docs.python.org/ja/3.7/

Note: On the current build, the /ja/ is not detected by the language switcher, as this *is* the step to add /ja/ to this same language switcher, this is the issue http://bugs.python.org/issue31146