2. .DIC and .AFF not in UTF 8 format

A good trick to convert the old encoding formats to UTF-8 is to use, for example, the Notepad++ editor for Windows.

Simply open the files with it, change the encoding to UTF-8 using the menu:
Encoding -> Convert to UTF-8 without BOM, so that accents appear well.

Then, use the Save As option and select "Normal text file (*.txt)" and it is done.

Please don't forget to change by hand in the header of the files, the word that has the old format, with the new one.

The headers with the font encoding are inside the files. See for example Version 2.4 (01/09/2007) of the Italian files:
- The Dictionary (.DIC + .AFF):
The .DIC has no keyword.

The .AFF has the following keyword:
SET ISO8859-15 -> Replace with SET UTF-8

- The Thesaurus (.DAT):
It has in the first line:
ISO8859-15 -> Replace with UTF-8