The issue might be because you are using iOS 18.0. There was a regression where many voices weren’t available via the Web Speech API that Readlang uses for the Read Aloud feature. This was fixed by Apple and in iOS 18.1.1 they were available again. There was a discussion about this at the time here: Read aloud is not working
(Note that the voice that speaks when you click a word to translate is a paid service provided by MS Azure instead of being generated on your device so this can remain working even though Read Aloud doesn’t)