Spoken Content: Random Changes in Language Used

I'm visually (somewhat) impaired and use several iPads to read to me. It used to work just fine, but with updates during the last six months the Spoken Content has gotten worse and worse all the time.

The problem is the same with each iPad: While reading (e.g.) a web-page to me, iPad randomly changes the language it uses when hitting a different formatting like bold.

Previously the text formatting did just change the voice slightly, but now I'm constantly interrupted by voice changes. All of the sudden I hear changing my favourite reader (Kate) to something utterly incomprehensible like a French or a Swedish voice pronouncing everything wrong.

Is there a way to lock the voice used with Spoken Content so that I can always hear my iPad to speak with the same voice and with that voice only?