Dual Language Super Simple

We need to set up a site with just two languages. We will be doing all the translation ourselves. Reading all the posts it seems clear that the simplest option is to create a second install of wordpress in a folder, with the same template, plugins etc, and have two small images in the top with the flags which leads to the respective languages.

My question is how to have it so that clicking on the flag for a different language takes you to the same page in the other language as you are on to start with. Would this have to be hard coded, or is there a bit of code I could put into the callback ( or whatever it is called ) that would do this for me?

This will be a static site. Using WordPress as a CMS simply for ease of simple edits and updates and the occasional change of theme.

Done a lot of C coding a very long time ago. Very little experience of html, css or wordpress.