As a Spanish colony and then a Mexican possession, both Spanish speaking authorities, how did California become an English speaking majority by the 20th century?
The Mexican American war ended with Mexico ceding a huge swath of what is today the Western United States to the USA with the treaty of Guadalupe-Hidalgo. United States settlers continued moving West, homesteading and occupying just about everything they found slowly. Then, gold was found near Sutter's fort in 1849 starting the gold rush.
The gold rush brought with it not only hopeful miners, but an entire support economy of inns, clothes laundering, food, general stores, you name it. It even brought immigration from China and some immigrants became rail workers and miners. California's population began to increase quickly in the following years.
Interestingly enough, California didn't enter the union as a slave state, but not because of magnanimity or equality. A vote determined it would be a free state in order to keep the large southern slave owning corporations from moving in, setting up gold mines, and out competing the other companies and groups that had setup claims. Motivated by greed it seems.
Overall, the days of the dominance of Spain and Mexico in the region were numbered when the USA won the war with Mexico. After that, an influx of white settlers began and really picked up around the gold rush. After that it's simple numbers.
-not a historian, took some classes. Be gentle if wrong