If you are using the multilingual extension in Drupal you are in for a big headache and time-consuming upgrade time!
What are the real issues?
The first issue is the multilingual extension change the way it keeps the content so you will have to be very careful how you edit your old content when you add translation otherwise you might lose the content altogether.
What happens is when you edit the old content it will show blank even though in the view it shows the old content just fine. So what you will need to do is to copy the old content from the view side then paste on the edit screen so go forward the content will be available for editing and translation.
The second issue is the language switcher it does not seem to filter the content according to the language chosen you to need to get "Entity Translation" extension so that you will get the language switch to filter the content base on the language that you choose.
You might have to edit every content of your site to get it properly save in the right place before you let your translator do the translation for your content otherwise you might be risking losing a lot of content!