Because it doesn't seem like this is a deal-breaker situation. As long as you stop cleaning her house and start making an effort to speak her language (I'm sure she'll appreciate any effort, especially if you instruct your DS to translate for her), you should be GOLDEN.
And really, if you're old enough to have teenage grandkids, you deserve a break! Let the teenagers learn how to clean up!
That being said, it *IS* rude to speak another language in front of someone who doesn't understand. We grew up bi-lingual and it was DRUMMED into us, how rude it was to my Dad. But he was understanding, if we were in a French-language situation, my Mother or one of us kids would translate for him. If my Aunt or Gpa *had* to tell a story in French, they would apologize to my Dad first, explaining that it didn't make sense in English. It wasn't that HE insisted that we only speak English around him, it was that we were polite about it, and considerate of him. Maybe if you make an effort with your DIL, she will appreciate it.