The DMV publishes the California Commercial Driver Handbook in English and in Spanish, and both are on dmv.ca.gov. Every CDL knowledge test comes from that handbook, so a Spanish reader can prepare in Spanish from the official text.
The tests
A commercial license is several knowledge tests, not one: the general knowledge test everyone takes, and one test per endorsement you need, air brakes, combination vehicles, passenger, school bus, tank, hazardous materials, doubles and triples. Each has its own chapter in the handbook and its own test at the DMV office. You get three attempts per test, and if the same test is failed three times the application expires and you reapply.
Training before the test
For an original Class A or B license, a Class B to A upgrade, and the S, P and H endorsements, federal law requires entry-level driver training first. The DMV checks it before the skills test and before the hazardous materials knowledge test. California also requires 15 hours of behind-the-wheel training, recorded on form DL 1236.
Practising in Spanish
All 2,396 CDL questions on this site are in Spanish, grouped by the eight tests and by handbook chapter, and every answer names the section it comes from. Each test has its own practice exam. The questions are written from the English handbook and translated, because where the two editions disagree, the English one is the one the test follows.
Practise the CDL tests in Spanish