Say that the Italians are competing in Eurovision, but they have a Brazilian composing the song and lyrics for them, or a Canadian as one of their musicians/singers. Does that flout the rules?
AFAIK, the only rule regarding nationality is that a performer can only represent one nation in a given year. Wikipedia sez; “No rule has ever been implemented to limit the nationality or country of birth of the competing artists; many competing countries with a small population, such as Luxembourg and Monaco, were regularly represented by artists and composers from other countries, and several winning artists in the contest’s history have held a different nationality or were born in a different country to that which they represented in the contest.”
As an aside, one interesting point I noticed in the rules is that live instrumentals are prohibited; all instruments must be pre-recorded and then mimed by the band onstage.
In 1988, Switzerland won by bringing in a young French-Canadian ringer with no Swiss background. She went on to have a fairly successful career.
As for writing, Tzvika Pick, Israeli 1970s pop icon (and Quentin Tarantino’s father-in-law) has written 3 Eurovision songs for Israel, one for Ukraine and one for Georgia.
To list some well-known cases where the musicians weren’t originally from the country they represented, Celine Dion was Canadian, not Swiss, (as Alessan mentioned) and Katrina of Katrina and the Waves was American, not British. But then, is it really that surprising that someone might move from one country to another to improve their chances at a career? I think of this as just another example of globalization. The professional sports teams of many countries and many sports are full of people not born there.