Finally found what I was looking for.
A company called "the Acapela group" has a great product called "Infovox iVox". These are high quality voices for the Mac. You can find the produce on the following page http://www.assistiveware.com/product/infovox-ivox
The basic principle is quite simple
- You download and install the voice manager for free!
- From the voice manager you can download and install more than 100 different voices in 31 languages (including French and Dutch !!!! Yes!!!)
- You can activate a 30 days free trial for each and every installed speech agent.
- If you want to purchase a speech agent after the trial you can do so for a very cheap price (14,99 for each agent with volume pricing available)
The voices are of pretty good quality and the price is very reasonable. One of the best value for money I found so far... and... best of all : these voices integrate well with Captivate as you can find them in the speech management window when converting a slide note to speech!