If it's not too much trouble please put an icon or some visual cue that the sentence you have to translate is an expression and should not be translated literally.
I've encountered a couple while practicing Spanish and it might be a nice addition to the system :)
I know, I hate it when my translation is marked wrong after I've struggled to figure it out. But on the other hand, making a mistake makes the expression easier to remember for me. Still, at least they could let us know afterwards, because sometimes even after the fact it's not clear.