I would like to know whether we can implement the hreflang tag for English audience in Germany like
hreflang="en-DE" or it should be always like "de-DE" and "de"?
So if you wanted to target French people in Great Britain then you would use
fr-GB and if you wanted to target English reading and speaking people in Germany then you would want to use
Google supports both ISO 639-1 and 3166-1 Alpha 2 format and therefore you can match any language with any region using the codes found in those iso's.
It should be mentioned that by using
en-DE you are telling Google that this page is solely for English people in Germany, meaning the page may not be returned for countries outside that region, if you want all regions across the world then don't use this method, or you could simply use
hreflang="en" if you prefer.