WPLANG is deprecated in WordPress 4.0. You may be interested to see the technical details.
Now, after completing the translation steps and putting your “de_DE.mo” file in the languages directory.
Go to `Settings > General `
Choose your language from Site Language drop down.
You are done.
Note: There is no need to set WPLANG constant now. If you download the latest version of WordPress from WordPress.org , you will notice that there is no WPLANG constant in wp-config.php file