I can see where you are confused. I tried this app and it seems to inject dictionaries into the built-in Dictionaries app. That's tricky.
I suggest you just use the built-in German-English dictionary, and maybe the German dictionary. The dictionaries that Apple includes with the operating system are high-quality dictionaries. You won't find equivalent content from some internet app. If you wanted something better than what Apple has provided, you will have to go directly to the major dictionary publishers. They have high-quality products that have better content that what Apple provides, but they will be expensive, hard-to-use, and will never work with the built-in dictionary.