A rosary seems like a really weird and pointless thing to non-Catholics, I’m sure. Heck, I think it seems really weird and pointless to most Catholics. You’ll be praying mostly three prayers: The Hail Mary (lots of em), The Our Father, and the Glory Be. If you look those up and learn them a little before you go, it might be more interesting than just kneeling and listening. (Oh, you’ll probably be kneeling for the whole thing.) The prayers will probably be said in a call-and-response form, where one person says the first half and then the whole group responds with the second half of the prayer.
The other thing you’ll hear is the proclamation of a set of “mysteries.” They might be Joyful Mysteries, Sorrowful Mysteries, Glorious Mysteries, or Luminous Mysteries. What these are are just a set of events in the life of Jesus and Mary. Between each decade (set of 10 hail marys), someone will proclaim the next mystery in whichever set you’re using that day. E.g., “The Third Sorrowful Mystery, the Crowning of Thorns.”
You’re going to almost certainly find it incredibly dull and repetitive. The thing that nobody ever tells you is that there’s a reason it’s repetitive, and that’s that the prayers aren’t really the important part. When you’re a Catholic, you know these prayers by heart, so they just come out of you without thinking. And that puts you (ideally) in a kind of sacred and meditative state. So what do you meditate on in that state? The mystery that was proclaimed at the start of that decade! With those Hail Marys are pouring out of your mouth, you have set the stage for imagining and reflecting on the Crowning of Thorns or whatever.
Last bit to know: Why are you doing it? Catholics believe in Purgatory, which is where people who committed sins in their life have to face the punishment for those sins before going to heaven. (The good news about being in Purgatory is that you know for certain you’ll make it to heaven eventually.) It’s like the prison where you have to do your time before you get to rejoin society. Well, the prayers and good works of people still living can basically be “donated” to a deceased person to shorten their time in prison. So when a bunch of people pray a rosary for someone who is deceased, they’re hopefully making it so that person will be in heaven that much sooner.
As far as the mass goes, there probably will be little or no Latin. Whether it’s in Spanish or English probably depends on who they expect to be there. If your grandmother and most of her family spoke Spanish, it will probably be in Spanish. There will be a very fixed set of prayers and songs, with readings from the Old and New Testament probably relating to death and life after death. The priest will give a homily and I assume someone will give a eulogy. The last half will be the consecration (when the bread and wine become the Body and Blood of Christ). You’ll do all the kneeling at this part. All the Catholics will process to the front to receive the bread (and maybe wine). If you’re not Catholic, you just need to stay in your seat; no one will think you’re weird or rude. When that’s over, it’s just another prayer or two (maybe the eulogies will be here) and you’ll be done.
You’ll definitely get to see the vestments, probably have incense dispersed over the casket, and hopefully it’ll be in a beautifully decorated church. Catholics like colors and statues and paintings and candles… all that sensory stuff. Since your wife’s grandmother is Mexican, I’d be shocked if you don’t see and hear about Our Lady of Guadalupe, the apparition of Jesus’ mother who appeared as a Mestizo woman to a native Mexican man in the 16th Century.
I know it’s a funeral, but I hope you enjoy the encounter with Catholic ritual and culture! When it’s done, let us know what made an impression.