flag multilanguage

Hello!
I created a copy of the website for use it as second language. In the root there is english, in the subdirectory /ja/ there is japanese. I would like to add somewere in the template (better on the header, or in the main menu bar) the flag with the link to the same translated page. How can i do it?

For example if i’m in the home of english, the link should go to the home in japanese, if i’m in the portfolio category in english, the link should go to the portfolio category of japanese, and of course the same from japanse to english. Is it possible to do someting like this? i don’t want to use multilinguage plugins. Thank you!