Hi,
I am not a technical person and I would like to turn my current blog into a multilingual blog.
Ideally, the translated blog texts can be added manually (to ensure a proper translation); several pages can be hidden from sight (some pages are for a certain country only); convinient to use (for the reader and for the webowner).

1. Is using plugin the best way? What are the arguments against duplicating the whole website (is it just inconvinience and SEO)? What is the best plugin?

2. I saw plugin WPML Multilingual CMS, but when I installed it, it messed up my blog. For example: all pages that weren't shown in the menubar are shown. Blog articles are missing, et cetera. Is this normal, how to fix it?

3. What are .mo and .po files? Where are they located?

4. Any tips on step-by-step instructions?

Sorry guys for this simple questions, please bear with me, thanks all in advance.

google is your friend - here is a good place to start your education: http://codex.wordpress.org/Multilingual_WordPress