Support for batch operations in 6.x

About $return :
actually you can put your stuff in the $result array whichever way you see fit - it's yours.
But I changed the syntax of batched operations since the earlier versions of the batch patch.
They now receive a 'batch context' array as a last param, that gathers results along with other info (mainly used for multi-pass operations)

function my_batch_op($your_arg_1, ..., $your_arg_n, &$context) {
  ... 
  // results are in $context['results']
  $context['results'][] = whatever;
}

The last batch patch contains more detailed phpdoc about that.

PS : i tested batching stuff inside _enable / _disable hooks, it works perfectly - when triggered by admin/build/modules form submit.
install script being (AFAICT) out of the regular FAPI form processing, having this work at install time probably requires some work on install.php to make it batch-aware (as I did on update.php). Being absolutely unfamiliar with this area, I did not venture there...

Batches can't be added or processed in form_alter : that's still form building time.
Batches come into play at form _submitting_ time : the natural place for a batch_set() is during a form_submit callback.

Your form_alter should add a new _submit callback to the form - then declare the batches in this _submit callback.

Lot's of TODOs in the code ...

Small detail: incorrect use of t(). You'll want to turn that $file into a %file.

drupal_set_message(t("Importing $filepath"));

Duplicate submission check works this way :
if the form you submit is the same, with same form values, than the last form the same user submitted, then it's considered a 'duplicate' submission, and the form is not processed.
$form['#skip_duplicate_check'] = TRUE; is therefore recommended for 'little' forms that are likely to be submitted with the same values multiple times, and / or when you develop / debug your forms...

That being said, I _think_ this area might change a little / a lot when the FAPI3 patch lands.

Comitted your patch to autolocale module with the following modifications:

- removed @ before include_once in .install file
- removed function_exists() call before include_once, because include_once itself ensures that
- added a TODO note to language addition
- added a proper path prefix value to language addition
- fixed some comments