|There is more information available on this subject at Spanish on the English Wikipedia.|
Spanish is a secondary language of the UNSC. Having its roots in Latin, Spanish bears many similarities to English and is used in various places throughout the UNSC, but has seen diminishing use due to the UNSC-wide adoption of English as its primary language.
Spanish is spoken mostly in areas where it saw prominence in earlier eras, such as Latin America and Havana. The language was also spoken on the UEG colony planet of Verge as evidenced by signs in the colony's subway tunnels.
Many members of the UNSC Marine Corps speak Spanish, including Corporal Perez and Private Mendoza. However, they also speak fluent English, as well. This would seem to imply that Spanish is still widely used.