FWIW: 1090MHz is, and has been, the only ADS-B standard outside the US since being adopted by ICAO in 2003. It became operational in 2006 with ICAO making it the official standard in 2013. The US delayed the mandatory ADS-B requirements until 2020. The US currently is the only nation in the world where 978MHz is an ADS-B OUT option. It is doubtful any other nation will adopt 978MHz as an alternative to 1090MHz.The one thing that I think is settled is that, outside of the US, 1090ES will be the ADS-B standard that all, or the vast majority of, countries will adopt when moving to ADS-B based ATC surveillance.
Since I often fly international, my aircraft has been 1090MHz ADS-B compliant since 20013.