Clever move if true!
Tesla could employ Spanish & Mexican workers and then swap them between the Spanish & Mexican factories - no language barrier!
Much easier to share knowledge between Berlin (Germany) and Valencia (Spain). While Berlin is 2000km far from Valencia and there are direct flights for barely 100 USD, Mexico is around 10.000km far, there are no direct flights (you have to travel to Madrid by high speed train, then take a flight to Ciudad de Mexico and then an internal flight to Monterey) and would cost around 2000USD. And of course 8h timezone difference.
Although both Spaniards and Mexicans speak Spanish, both Spain and Germany are in European Union, we share timezone, we share external borders (so it's a domestic travel), we share laws and, of course, we share continent (Europe). No good deal moving people between Mexico and Spain. We can use phone.