Installing two different language version of WordPress

I am currently running an English page with some Japanese language in it. I want to go fully bilingual and have all content in both language and give users the ability to toggle in between the two languages. I know this can be done, since I have seen WordPress sites that do this.

I currently have WP in English installed in my sites root folder. My plan is this: Create a folder inside the root folder called “JA” and install the Japanese language version of WordPress in this. I would then access the Japanese cpanel by going to http://www.site.com/JA/admin/. Then hopefully find a way to link in between the two with “See this page in English” and “See this page in Japanese.” I think if the post slugs are identical in the Japanese and English versions, it should be easy to do this, but I am not exactly sure how.

My question is: Will installing two different language versions of WP, one in a subfolder, completely screw up my site? Do I need to set up subdomains? Also, any advice on how to easily link between the posts in each version would be greatly appreciated.