I have done this in an older version of Ubuntu a few years ago for a Doctor - English, Dutch & Chinese - but cannot seem to get it to happen with LMDE Xfce.
At the moment I need to be able to have two logins - different usernames - so 1 user can have one Desktop for one language and another Desktop for other language.
The languages are shown as already installed and I have installed additional packages via synaptic but still only get the options of US and En English at login.
One problem I have noticed is that to install additional packages for the second language is that Synaptic and Software Manager want to remove the English versions already installed - very weird!!!
Can any explain how to do this.