By default the languages a page is translated to are always displayed as autonyms. In my situation I needed them to be shown in the user interface language. I accomplished that with the following one-line hack in PageTranslationHook:languages():
...
foreach ( $status as $code => $percent ) {
// Get autonyms (null)
$name = TranslateUtils::getLanguageName( $code, $userLangCode ); // #custom: Display language names in selected language
$name = htmlspecialchars( $name ); // Unlikely, but better safe
...As I'm probably not the only one who would need that behaviour it would be nice to have this configurable.