How to reuse Preferred language select?
In CiviCRM, custom fields have the ability to re-use options from another custom field, but not from a core field like Preferred Language. So this is not currently possible via the UI.

2 Solutions come to mind, both using small amounts of custom code:


Write a short script that uses the api to fetch all languages and then copy them into the options of one of your custom fields. Then re-use those same options in 3rd, 4th language fields.
Write a tiny extension that implements hook_civicrm_customFieldOptions and populates those options dynamically by outputting the languages in the system.