well then, What do you think would be the most likely effects if a Crusader State were set up in Tunisia around say, the mid 12th century, and kept in Christian hands until the present?
(possible, I'd say, considering that its most likely base of support, the Italian maritime states, are much, much closer then to the Levant)
That's a rather interesting idea, actually.
However, I doubt wether such a state would become completely Italianized.
Assuming that this state would be a pretty large, stable and succesful Crusader kingdom, then at least during the first century, the Christian population would mainly consist of merciants, soldiers and colonists that are maily from Italy, France and Spain. There will be a number of local converts during that first century, but their numbers will not be large enough to make a real difference.
The Christian population would be very mixed, and the main language of the Christian population would propably be Lingua Franca or a similar mixed language, and this language would then most propably become the official language of the kingdom at some point.
And in case this kingdom survives and manages to hold on to a significant part of Tunesia until the modern age, then in time, a good part (propably the majority) of the Arab/Berber Muslim population would convert.
But these converted Muslims would either assimilate and adopt the official language of the kingdom (Lingua Franca, or a language that is derived from that), or they would continue to speak an Arabic or Berber dialect, not unlike the Maltese.
And if the majority of the converted Arabs and Berbers assimilate into the colonist population (which is quite likely), then this will lead to an influx of Arabic, and to a lesser extent, Berber loanwords into the language of the colonists, which will make the local Christian culture even less Italian (even though it was never completely Italian to begin with) and more uniquely Tunesian.
On the long therm, the result would be very interesting, as the kingdom would most propably have develop its own unique language and culture. But the odds are that the kingdom would not become Italianized.