Hello John,
I've configured a multilingual system based on Debian Jessie last month.
The steps I've followed are:
1) Install the language pack of your targeted languages like
"task-english", don't miss to install the language pack for Firefox,
Thunderbird and LibreOffice if needed.
2) Enable the locales of your users and move the default locale to
"None" with :
sudo dpkg-reconfigure locales
3) Log in with one account
4) Edit the file "~/.dmrc", replace the locale with the locale you want.
5) Do the strep "4" for all the users
You have to reboot your computer after applying the configuration to
avoid cache. IMO, you should login into all your users to ensure that it
works or test the procedure on a VM first.
Could you let me know if it works correctly for you and what issues do
you have to be sure that my Jessie procedure continues to work on Stretch ?