How can I create a select list with all languages? I need to have a field on a form, where the user can choose from the set of world languages for an event.

It's going to be an Autocomplete term widget (tagging) field.

How can I reuse the inbuilt language list in Drupal for this purpose?
there is the _locale_get_predefined_list which return you an array.
so you need to do the field yourself.

you also could try language_field, you should be able to use that directly in your form.