Duplicate english folder and rename folder to your language name. Open each .php file with your favorite text editor and translate the content of each constant. For example;
On this example, you should change only the text on the right side: Home, Special Offers, Contact, Sitemap...
When you finish translating, login to administration. Go to "System/Localisation/Languages" and click "Insert" to add your language to your system. Enter your language details and click "Save" button. That's all.
If you want to set your language as system default, in the administration panel, go to "System/Settings", click "Local" tab and change the value of "Language" to your language. Click the "Save" button to save your changements.