Spanish is the language in Mexico, or at least the Mexican version of it. With exceptions, English is spoken in the tourist areas of Cabo San Lucas by most, including in the downtown shops and restaurants. Fluency in English tends to be related to pay grade, so, for instance, the maitre d’ and the waiters will typically speak English while busboys and cleaning staff typically will not (just like in the Southern California!).
There are some odd exceptions. The car rental agency offices a the airport all have English speakers, but the local car rental offices sometimes don’t. Gate security, even at hotels, don’t always have English speakers, which seems to be fixed by waiving through gringos.
If you are adventuresome and want to travel outside of the tourist areas, you probably should have a Spanish speaker with you.