I don't know if it will work for everybody, but after you set all the languages you want in the Lang & region,
- go to Settings > Time & Language > Language & Region > Administrative language setting
After you click on the on the Administrative language setting, it should open a new window,
- On the new window, in the "Welcome screen and new user accounts" section, click on "Copy settings..."
- make sure the setting in Current user box is the one you wanted, then just click on the 2 tick boxes below (Welcome screen and system accounts, New user accounts) then hit ok.
After all those steps I did this just as an extra precaution.
- go to Settings > Accounts > Windows backup
- click on the "Remember my preference" (expand the thing, not disable it entirely)
- Turn "Language preferencees and dictionary" off
Then restart your PC. Every unwanted language should disappear. Sorry for the lack of images. This thing seems not to want to accept my screenshot :P
edit 1 : reformat