I remember when they announced it,
I remember hearing them call him Franciscum… and when he talked about how/why he selected his name, he said it in Italian, “Some people wanted to know why the Bishop of Rome wished to be called Francis.” (and he says Francesco… here is the video, you can see it at time 15:30 of the video on demand):
radiovaticana.va/player/index_fb.asp?language=it&tic=VA_HGWO2CQF
Anyways, I think it is customary and acceptable for each country to call him by his name in their language. Our Church is universal, our pope is universal

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ETA: Here is the text from when they first announced him:
Annuntio vobis gaudium magnum;
habemus Papam:
Eminentissimum ac Reverendissimum Dominum,
Dominum Georgium Marium
Sanctae Romanae Ecclesiae Cardinalem Bergoglio
qui sibi nomen imposuit Franciscum
vatican.va/holy_father/francesco/elezione/index_en.htm