"We ask the guard to call the police."
Translation:Vi ber vakten ringa polisen.
I'm not native but I think they would understand anyway, and for the second question; i think some of them would, but mostly not, they tend to let it pass, even just speak english with non-native because it's easier for everyone, even if you keep asking them to speak swedish with you (at least thats my personal experience). Also, I've seen this a lot in many occasions, no matter which language you are speaking, most people tend to not say anything when they hear a mistake because they are afraid to offend the other person.
I just recently had a discussion about it here: https://www.duolingo.com/comment/7075383$comment_id=7078343
You can find there some good rules when to use att and when not.