So here's what I've done:
- added an option to the array in qa_admin_language_options()
'en-US' => 'English (US)',
- initialized the options array to empty, instead of with '' => 'English (US)'
- after the loop to detect existing language options (subdirs) for the select input, check to see if an en-US option was found, otherwise add the old '' option
if (!isset($options['en-US'])) $options[''] = 'English (US)';
So, if there's an en-US subdirectory in qa-lang, that's what the 'English (US)' select option will point to, otherwise it will point to '', using the default files in qa-include.
Seems to work. Would be nice if this or an alternate solution were rolled into a future iteration of Q2A.