(1) You do not *have* to copy all the screen structure files in order to provide alternative text for screen labels.
(2) If you put the file 'language_text.inc' in the 'en' subdirectory then that is the English translation. What languages are you translating from/to?
(3) There is a difference between the column_id and the label text. For example, in my Example application (included in the Radicore download) is a screen 'person.detail.screen.inc' which is used in the 'Enquire Person' task. This uses the label 'First Name' for the column 'first_name'. To translate this into another language your 'language_text.inc' file must contain an entry such as
$array['First Name'] = 'Foo Bar';