I created a multilingual blog like fi.mysite.com/blog and en.mysite.com/blog. Everything seems to work well, but when you post a comment, you are redirected to mysite.com/blog/wp-admin/ page.
Permalink settings: Month and name, http://mysite.com/blog/2014/03/sample-post/
Front page: non-static
Polylang settings: URL modifications, The language is set from the subdomain name in pretty permalinks, http://en.mysite.com/blog/my-post/
There is only one installation and DNS records are configured to make all the related domain names (fi.mysite.com, en.mysite.com and mysite.com) to point the to the same IP address.