There are two altars at the Sagrada Familia church: The Main Altar and the Crypt Altar. Masses are held at both.
The international mass is at the main altar. You can enter using the entrance on the Nativity façade (Carrer de la Marina).
Masses are also held daily in Catalan and Spanish at the Crypt of the Sagrada Familia. You can gain access to this altar via 318 Sardenya Street.