Welcome, Imani! You have a choice of three Catholic churches, which I guess are more or less the same distance from you: Our Lady and St Philip Neri in lower Sydenham (208 Sydenham Road, near Home Park); St William of York, Forest Hill (Brockley Park, just off Brockley Rise, near the S. Circular); the Resurrection of Our Lord, upper Sydenham (Kirkdale, on the corner of Sydenham Park), which is my church.

The only Catholic church I can find in the Southwark diocesan directory which has masses in Spanish is St George's Cathedral, in Lambeth Road (opposite the Imperial War Museum) which apparently has a mass in Spanish at 1.00 pm on Sundays.