Just place it inside locale folder.
For eg. Russian language can be added as,
locale/ru/ru.php
Where ru.php file contains the translations,
then in sites/default/constants.php
edit the line
define('LOCALE', 'en_US');
Change en_US to ru
Just place it inside locale folder.
For eg. Russian language can be added as,
locale/ru/ru.php
Where ru.php file contains the translations,
then in sites/default/constants.php
edit the line
```
define('LOCALE', 'en_US');
```
Change `en_US` to `ru`