Sorry about this. When I added this feature there were some decent quality voices available on iOS, and I figured that over time Apple would add more and/or improve them. But with the last two iOS updates it seems that support for the Web Speech API (what Readlang uses to play the speech) has got worse 
. With iOS 18 there are no decent quality voices available in Spanish (and probably other languages too). Sorry about this. Hopefully either Apple will fix this and/or I’ll come up with another solution in future (e.g. native iOS app or server generated voices)
The best browser right now for Read Aloud is MS Edge on desktop which has some wonderful Azure-powered voices available.
(If you happen to have mp3 audio files or know how to use a TTS service to generate them you could try out the new mp3 upload feature: NEW mp3 audio upload with auto-transcription!)