We should select French as the default voice now if you selected it during setup. + leberly as FYI.

Chrome 63.0.3233.0 (Official Build) canary (64-bit) (cohort: 64-Bit)

Resolving this bug as verified. 

Change language into French, reboot. French voice started automatically and read in French. (My high school French class came in handy!)

Changing back to Available since I can now repro this issue. Just after I updated this bug, ChromeVox stopped speaking in the French voice and had the English voice trying to speak French. It was distinctly different from the French voice that had just been speaking. All I did was tab to another part of the screen.

ChromeOS Version 63.0.3238.0 (Build officiel) canary (64-bit)
Firmware Version Google_Samus.6300.174.0

I dug into this further and it looks like language switching is behaving normally except that the login screen is not respecting language choices. Here are the steps I used: 

# Settings > Advanced > Language Options checked that English (United States) says underneath "Chrome OS is displayed in this language", I also previously installed several other languages and they are listed afterwards: English, German, French. 

# Accessibility > Settings > Manage Accessibility Settings > Open Chrome Vox options > Voices is set to Chrome OS UK English 

# Turn on ChromeVox
# Shut down laptop and turn it on again
# ChromeVox is not speaking on the login screen
# Press ctrl + alt + z and US English voice is heard 
# Login
# ChromeVox starts speaking in UK English 

I then examined what happened when I set both languages set to French gradually. 
# Setting first just the ChromeVox to French, it started speaking the English words in a French accent. 
# Setting the system language to French, it disabled the French ChromeVox language and reverted into English. 
# Restart
# ChromeVox English is speaking on the Login screen in English
# Settings page is now in French and ChromeVox speaks in French. 
# Tabbing around and looking at new pages, things are consistently in French