I had the same problem with French TTS and as Osiris4isis say If you have a TTS alert working for other alerts, there is no reason why this would not work for "average speedcams".

The problem is not located in the Sys.txt but at the dictionary level.

You will need to check the dictionnary.voice file located in i18n.

Check the "Caution traffic light camera" line (use the Notepad ++ and the search function and do not extract the zip)