What does not exist? You are telling that WP_LANG does not exist in wp-config.php file? If that's what you mean, you should definitely re-install your wordpress files because 4.0 version is outdated.
This path was always there and this path was made on our side only. It says only in which folder on our computers, files were generated but this path does not have any impact on how theme works. So you should not bother with this path as it does not mean anything.
Probably you did something wrong and that's why it does not work. Or like we told you above, something is wrong with your wordpress or theme files because it is impossible to do not have language function under wp-config.php file. We are not your website developer so we don't know what could be wrong. You can switch to any default theme to see that our theme is not the problem.
Comments
did you changed language under wp-config.php file as well?