We are aware of that we lack support for imperial units here and there. Though it's not always we are aware of the specific locations, since we use metrics ourselves.
Generally, everything is supposed to go imperial if you check that checkbox in settings (clicking your username in the menu). If this doesn't fix the measure tool, then we do have a bug. I will bookmark this one so that we can verify it and later on fix it. (I wouldn't be surprised if we have missed it for the measure tool)
