Accented letters won't break retail compatibility, so feel free to use them. It's just the fonts that require FSO. In fact, no languages other than English, German, French, and Polish can be made to work in retail, at least not with FS2's localization system, so an Italian translation will definitely require FSO.
The only characters that you need to be careful to avoid using are semicolons, double quotes, and dollar signs (the last of which may be used only as part of color tags). All other ASCII characters as well as all non-ASCII characters in the list that I linked to are OK.