Did you ever fix this issue? I have the same issue, but inverse. I bought my iMac mid 2011 in UK and since upgrading to Sierra have found the mechanical disk makes the machine sooooooooo slooooooooow. I moved to France a few years ago and so I use iConcept for Mac service. I took my iMac there yesterday for an HD upgrade to SSD - Mac is like a rocket now, however, they put a fresh install of Sierra on there..... in French. Now I speak (fluent) French, but on a computer, and with that crazy keyboard layout, I'm sorry - it just does not work for me. I have spent about a day now setting everything back up, copying files back from back up (not time machine as I didn't want mistakes made 6 years ago back in again) and as you found, everything in French including the keyboard! I had to bring up a picture of a Fr keyboard on my MacBookAir to see which keys to push for passwords.
You can get around most of the keyboard and language issues in Preferences, but not the ones you are left with. If you the Apple site for 'Change the language used at the login screen on your Mac' the instructions are great and they work. You will see the login, shutdown etc. in language of your choice. You can also choose your keyboard - be patient - it takes time each change. However.... it does not fix the bizarre mixed text log out dialogue box issue. Mine is in French with English reminder about 59s in the middle.
Anyone know how to fix this?