I found the MUI version on Adobe's FTP site. I wonder why it is not among the choices in the standard download page...
Now, it seems to be a package containing both core and extra languages (not what I'd call a "MUI" pack), and it also seems you cannot choose which extra languages to install and which ones not to. If anyone knows how to do this, that would be nice, even if, to be honest, the size of this MUI package is very similar to standard EN's - this means you wouldn't have a real gain removing some languages...
You can use below mentioned command line to install MUI Reader with selected languages
msiexec /i "Full Path to Reader MUI MSI" LANG_LIST=ENU,FRA,DEU,JPN
Thanks! Indeed I hadn't thought of checking the .msi properties via the Adobe Customization Wizard or Orca... still suffering of new year's side effect, maybe.