Would they be speaking in mostly English?
I would think that amongst their own friends and family they would be speaking Spanish about 90% of the time. Correct me if I am wrong. Mexican friends that I have had spoke Spanish most of the time when they were with their families.
I guess this was just a choice that the writers made to appeal to a wider audience.
"How many breakfasts did you have this morning? Looks like you had four, maybe five."