Thanks, very helpful indeed.
I just got TTS working. This is a howto.
Install your TTS-App.
The problem:
TTS-options are not existent in the system settings.
To set them we need a third party app, like
FBReader TTS+ Plugin.
Install it, start it, click the middle option.
Now click: Change Voice or Language
(Sometimes the options are shown and sometimes not, try again if not visible)
Select: Use only the system default voice
Select: System TTS Settings
and you are in the System TTS settings.
Now select all according to your preferences and check if TTS is working by listening to an example. If this is working, you can now use TTS systemwide (tested with Coolreader, AlReader, Fora Dictionary), not working with the preinstalled FBReader.
Edit: Created an APP with Tasker to have easy access to the settings within this APP. No need for "FBReader TTS+ Plugin" anymore.