The FreeDict project aims at providing free (open source) dictionary databases, to be used both by humans and machines
The official home is at http://freedict.org. This page details how to get and use the dictionaries.
All dictionaries are encoded in TEI (version 5) which is a flexible XML format to encode human speech. The FreeDict project provides dictionaries but also style sheets to convert the TEI databases into human-readable formats.
In the past there were a variety of output formats, but at the moment only the dict format is supported.
As mentioned above, please have a look at http://freedict.org for instructions. For Debian/Ubuntu/Mint, the installation is really easy. Just search in the package manager for packages containing "freedict", i.e. for the English-German dictionary:
sudo apt-get install dictd gnome-dictionary dict-freedict-eng-deu