add test and solution for giving multiple language separated with comma

I realized when entering "de,en,es" in the interactive installer, I got an CMS install with just exactly that single language.
Here's the commandline fix for this.
I didn't yet check if and how it works interactively, as I do not see yet how to use the install I checked out to actually run the installation from there(I'm a  newbie with aldryn and this type of development).
Also, as the TODO says, in interactive mode, it might happen people enter "de, en, es" so I'd like to remove whitespace, too. doing that in a following PR if that one get's pulled.