That's true, when you switch the app it goes back to the main screen. There's a list of the last 100 things you looked at which makes it pretty easy to go back, though I don't find myself using it that much.

Do you mean that you can copy something in any application and Lingvo will pop up a translation? That is pretty cool! I don't think that's possible with iOS, but there are some apps that let you do that with Safari only. Look up "safari translate" in the app store. I haven't tried them personally.