What is the correct way to call wordpress functions from another script?

So, I hope this is the correct section.
It’s very frustrating, I swear I have searched everywhere online and I can’t seem to find similar problem to mine.

I simply need to use some functions included in :
wp-admin/file.php
wp-admin/media.php
wp-admin/image.php

What I am doing is

<?php
require('wp-admin/includes/media.php');
require('wp-admin/includes/file.php');
require('wp-admin/includes/image.php');
...the rest of the script
?>

I’m not including the rest of the script because the error is already here :

Fatal error: Call to undefined function __() in /membri/infomjmotor/wp-admin/includes/file.php on line 13

Okay, if we go looking for the line 13 of file.php what we’ll find is a series of functions that actually start with “__(“. What do these two underscores mean? The problem seems to be within these, because if I remove them the error changes.

What am I doing wrong? This should be pretty simple…Including the scripts I need and using the functions I need. But I think WordPress doesn’t like this. I can’t find anyone having the same problem, I have not upgraded, this is a fresh install.

I tried reinstalling, even different versions, the problem persists even in a fresh install.

Thanks.