How to prepare SimplePie_Item’s get_date() for l10n?

Best way would probably be to use the built in Internationalization function in WordPress. This will then translate according to the language pack being used.

The “U” character to a date() call gives the unix timestamp, which is just what you need in this case, for passing to date_i18n().

For example:
echo date_i18n('j F Y @ g:i a', $item->get_date('U'));

The months and such will be translated accordingly. If you want to add the __() wrapper around the date format string, that will work just as well.