forms – Lookin for the the correct way to alter a field

I am trying to find the correct way to alter a field on a multilingual Drupal 7 site. I am using multilingual nodes but not multilingual fields.

I can alter the field with:

$form('field_uploaded_files')('#prefix') = 'This is a test';

This works without a hitch.

If I try and then do something slightly more complicated like

$form('field_uploaded_files')(LANGUAGE_NONE)(0)('#upload_validators')('_app_magic_number_validator') = array($form_id);

then it does not work at all. I have tried with and without (LANGUAGE_NONE). I have tried with and without (0).

I am at a loss for why this is not working and would appreciate any help.