I’ve done some testing myself and here’s what I’ve found:
On iOS
I’ve tested it for Spanish on iOS (version 17.6.1) in the following browsers:
- Safari
- Chrome
- Edge
- As a full screen PWA
And in all of these I have the following two voices available:
- Monica (es-ES)
- Paulina (es-MX)
If these don’t appear, you could try checking here to see if they are downloaded to your device and to download them if necessary: iOS Settings App → Accessibility → Spoken Content → Voices → Spanish. Unfortunately however, installing more voices here doesn’t seem to make them available to webapps like Readlang, but perhaps if you don’t even have those two voices I listed above then installing them might make a difference.
On Android
I only have one working Android device at the moment, which is an entry level RedMi phone running Android version 9 PPR1.180610.001.
On Chrome, initially I had no voices available in Spanish, but then I managed to install them at: Settings → Accessibility → Text-to-speech output → Preferred engine (make sure it’s set to Google Text-to-speech) → cog icon → Install voice data → Download your desired Spanish voice
Readlang’s read-aloud feature then worked, but doesn’t sync to the text. Just as you first reported. It’s likely that the relevant part of the web speech api on Android Chrome isn’t fully implemented. This bug report seems to confirm this: Chromium
On Edge - same result as you, no voices available 