Using the past 2 generations history, shiny legend events have left Japan (Rayquaza, Mewtwo, Xerneas, Yveltal, Zygarde, Tapus), but shiny mythical events have been inconsistent (Jirachi, Genesect, Arceus, Diancie).
I say chances of a Shiny Necrozma leaving Japan is mediocre, but the existence of TCG card (and it getting translated) would boost it up to likely.