1. Enable these module : Content type translation, Content type translation
2. Edit the content type which you want to change the language
For example, here we modify the "Pages" content type.
a. In the "Workflow settings" block, enable the "Multilingual support"
b. In the "Multilanguage options" block, you can also select the option you want.
3. Edit the content node
Now when editing a "Pages" type content node, you will see that there is a selection called "Language".
Have fun! :)