It works by replacing the files of an english SpeechVoice folder by the french SpeechVoice counterparts - so to say using an english hull and fill it with french content - and some further adjustments:
I have done the following as root user.
Download your preferred french voice. Un-tick all voices you don't want to download.
Make a backup copy of an english voice (accidentally I also d/led Fiona) with Terminal.app or make a copy with the Finder and rename the folders adequately:
cp -R /System/Library/Speech/Voices/Fiona.SpeechVoice /System/Library/Speech/Voices/Fiona.SpeechVoice.backup
Also make a backup copy of the french voice:
cp -R /System/Library/Speech/Voices/Aurelie.SpeechVoice /System/Library/Speech/Voices/Aurelie.SpeechVoice.backup
Now move the content of Aurelie.SpeechVoice.backup to the respective folder(s) in /System/Library/Speech/Voices/Fiona.SpeechVoice
Then open /System/Library/Speech/Voices/Fiona.SpeechVoice/Contents/Info.plist (which is in fact the moved Info.plist of Aurelie)
and /System/Library/Speech/Voices/Fiona.SpeechVoice.backup/Contents/Info.plist with a text editor.
Replace the strings of the following keys in the file /System/Library/Speech/Voices/Fiona.SpeechVoice/Contents/Info.plist by the strings of the english original (I don't know if this is really necessary - I just did it):
Save the the Info.plist, quit all apps, delete the now empty folder Aurelie.SpeechVoice.backup (and all other unwanted SpeechVoice folders), log-out root, log-in to your common account and disable root (for security reasons).
In System Preferences open Date & Time -> Clock -> Customize Voice and choose the "doctored" english voice. Keep all other speech/language/region settings like they are (some uk-english probably ;-)).
The only cave-eat: Sometimes - or always and the VM in which I tested the modification just skipped the beginning - the voice says "It's dix heures quarante cinq" instead of "Il'est (or C'est?) dix heures quarante cinq".