I have been using the WhiteStork dictionary on my overseas travels.

For complete install instructions, see http://mdictionary.garage.maemo.org/userguide/

Here's a condensed version (and some notes of my own) that I have used:

The WhiteStork Multilingual Dictionary has been built with three modules, dependant of each other: WhiteStork Core (internal application management), WhiteStorkGui (graphical user interface) and WhiteStorkPlugins (parsing engines).

In order to install the application take the following steps:

1. Download the 3 binary packages from WhiteStork website -

https://garage.maemo.org/projects/mdictionary/

2. Open the file manager on the N800 and install by double clicking on each of the downloaded files in the order they were mentioned above.

Download the dictionaries from

http://xdxf.revdanica.com/down

You will probably want both the English-to-Italian, and vice versa. Note that I never was able to figure out how to extract the files using the N800. I had to download the files on my Windows machine and use 7zip to extract the files. 7zip is like Winzip, but with support for some other formats. Download it here

http://www.7-zip.org/

NOTE on dictionary names - the files MUST be named "dict.xdxf". Also, the directories must NOT have any spaces in them.
Example (valid): English-Italian/dict.xdxf
Example (invalid): English Italian/dict.xdxf
Example (invalid): English-Italian/foo.xdxf

Once everything is installed, you should be able to open both dictionaries at the same time and do your word lookups.

Good luck!