Show to test on: https://thetvdb.com/series/amika
Currently running $response = $this->client->series()->allEpisodes($tvDbId);
Which returns the eps, but their names are null. Because they have no English translation, as it's a Dutch show.
If I run the API call manually https://api4.thetvdb.com/v4/series/169161/episodes/default then I get episodes back with their Dutch names.
I think now you need to specify a secondary language, but you can't know that beforehand. It basically does try to get English by default always by adding the /eng route to the URL series/169161/episodes/default/eng but without that, it returns the default show language.
Perhaps a flag somewhere that if enabled, and if title is null, it merges in the original?
[EDIT]
I now see that only the allEpisodes() method forcibly adds 'eng' as the language parameter to episodes(). Grabbing a single episode always returns the original title it seems.