It'd make sense for SEGA to start publishing Yakuza 0 after Yakuza 5 is out of the way. A lot of things that was holding that title from coming over (Biggest in series, Haruka dance segments, SEGA's restructuring issues.) isn't present in Yakuza 0. If they were happy enough from the sales of Yakuza 3 and 4, however little they might have been, than I don't see any reason why they wouldn't greenlight the sixth installment to be localised, especially after SEGA Sammy's recent comments that next generation consoles are doing favorably in the West.
I mean, it's not a foregone conclusion, they'd/Sony still would want to see some good sales from the fifth game but it's not as far fetched anymore.