Calls to __() in an AJAX function are not translated even after I set the lang parameter in the AJAX URL.
It appears that Sitepress is not detecting the lang parameter properly.
I've found a workaround, but it seems cumbersome.
global $sitepress;
$sitepress->switch_lang($_REQUEST['lang'], true);
load_theme_textdomain( 'mytextdomain', get_template_directory() . '/languages' );