It's not nationalism it's just been favoritism towards Valverde in the 2015 Tour.This narrative of nationalistic decisions against South Americans does not make any sense at all.
Movistar had a lot of GTs in the last 10 years where a South American was their clear leader or at least co-leader. They are even scouting these riders emphatically, as the Latin American market is very important to the sponsor (maybe even more important than the European one). So why should they act against the sponsor's interest?
In fact, there have been plenty of incidents where Movistar sacrificed the chances of some Spanish riders, as the South Americans appeared to be more likely to get them victories (Landa riding for Carapaz in Giro 2019, Carapaz also announced as their leader for Vuelta 2019 before his crash, Soler forced to wait for Quintana in Vuelta 2019, Valverde riding for Quintana in Tour 2017 despite being stronger...).
They also did not stop MAL's attack on Gamoniteiru, even if he gained time on Mas.