The site is displaying this error message: Fatal error: Cannot redeclare comment_exists() in ZS on line 27
This problem did not happen because of WPML, but how the client server is configured.
The "include_path" option in the php.ini configuration file was using a path other than the default.
For this reason, it was necessary to edit the theme's functions.php file and add the constant __DIR__ so that the included files were done with the full path, instead of the relative path.
This was not working because of the configuration of the php.ini file:
Adding the complete path __DIR__, it worked:
include __DIR__ . '/includes/comment.php';