Hi I had the same problem because I write articles in both Greek and English.
Check the wordpress.org instructions about how to add language to your blog.
Extra help in Greek here
First of all you need to clarify in which language you need your main functions to be (post dates, dasboard language, etc). I would suggest you keep the English for the main functions which means you skip the second step as the instructions say,
Then you have to change the unicode of your server database. I would not advise you do try it by yourself. It is very very dangerous, you can loose files. Consult a web designer or ask it from your host customer support (that's what I did and now it works great). However it you decide to try it by yourself backup your data before any change.
Just an example