Set Italian (it) as fallback for Corsican (co)

As requested by both active translators:
<https://translatewiki.net/wiki/Thread:Portal_talk:Co/Fallback_to_Italian>
One is French-speaking and the other Italian-speaking: turns out
this fallback is not controversial at all.

Change-Id: I050c84b0cd435379dd5ecbf22d2128101775bf12