on my PC I found a folder 'TTS' (1,9Gb). This contains the global folder LoquendoTTS/ with a large number of international files. The folder and the APK on Android device/primary folder, the APK installed, voice output switched from google to LoquendoTTS and German Katrin selected; flawless TTS voice visible in iGO.
Everyone has to test for themselves whether and to what extent it fits with other countries/voices.
Spoiler: download
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