voice modul 'Default Text to Speak' speaks terrible english (pronouncation)

First off, sorry if this should turn out double posted, but there was a problem with the page not loading after I did submit the first time, and I do not see the first version in the thread list

Hardware/Software
TS3 version 3.0.18.2, Qt version 5.5.0
Windows 8.1 GERMAN 64 bit, 8 GB RAM (i think the german OS is the main reason for the problem probably)
Sound device on board (Asus Laptop GV75X-CV012H) RealTek HD driver version 6.0.1.6728
using Headset PlanTronics GameCom 780 driver version 8.1.8.2164

Problem description
when using the 'Default Sound Pack (Female)' TS uses fine english, perfect pronouncation.
when using the 'Default Text to Speach' sound pack then the english with terrible pronouncation. I would liek to use this settings so it tells me the names of people entering, not just 'user'. Even worse, if the username contains special characters (e.g. 'underscore') it uses the german names for that special characters (in this case 'Unterstrich'). And this even when I switch the TS language to english (from 'system dafault' or 'german', does not matter). This is why I suspect that the OS language is the problem.

Expected way of functioning
well, when I switch the language to english, it should really use english (and decent one), shouldn't it?

voice modul 'Default Text to Speak' speaks terrible english (pronouncation)