So does anybody have any clue as to why "De" is translated as "The" in this sentence?
This happens when there is an adjective and a definite noun (i.e. a noun which in English takes "the" instead of the indefinite "a"), De/Den/Det [adjective in -e form] [noun]
I guess you are using the mobile app only. Refer to the "tips and notes" of the lesson in the web version where you have a complete explanation.
so when do you say, "blå skoene"? wouldn't it mean the same thing, or what's the difference?
You wouldn't, it wouldn't make sense to say it like that (as in it doesn't follow Danish grammar, it does make sense in that it would be a logical guess, but it is still an incorrect construction)