Hello, stantokm! So let's un-pack this a bit, since I think there are a couple of issues.
First, missing your home parish's Mass was not a sin. You had intended to go, you believed there was a 7pm Mass, but you were in error. It's an accident, or a misunderstanding, not a sin. But now you know you need to be more cautious and confirm Holy Day Mass times well ahead of time ;). So if you fail to do so next time, missing Mass as a consequence may indeed be sinful.
The more troublesome bit, I think, is the Spanish-language Mass that you could have gone to, but chose not to. I know that attending a Mass in a language you don't know isn't ideal, but it's still attending a Mass, which you are required to do on Holy Days of Obligation. So yes, you should have gone...and failing to do so was sinful.
Now, is it a mortal sin? In order for a sin to be mortal, it must be grave matter, you must have full knowledge, and must have full consent of the will. Missing Mass on a Holy Day of Obligation is grave matter, and I'm pretty sure you had full consent of the will (I assume nobody held a gun to your head and prevented you from going ;)). So the question is whether you had full knowledge that you were obligated to attend Mass, and that attending the Spanish-language Mass would have fulfilled that obligation. If yes, which I'm guessing is the case, then the sin was mortal. If no, then it was more likely venial. But only you know for certain.
Either way, I agree that you should definitely bring this to confession. Even if it's venial, which isn't strictly required to be confessed, it never hurts to bring all of your sins to the Sacrament! Go get yourself washed clean and begin anew.
And in the future, don't be afraid to drop-in on a Mass that isn't in your native language! My parish has a Vietnamese-language Mass on Thursday evenings, and sometimes I sit-in (and there are often one or two other non-Vietnamese folks sitting in too). I don't know Vietnamese, but the Mass is the same! I can follow along pretty well, even when I don't have a Missal or Missalette handy.
I find it to be a nice little reminder that we're part of a world-wide, universal faith. All Catholics, all throughout the world, are practicing the faith in basically the same way...reading the same readings, praying the same prayers, receiving the same Sacraments. I love it!