As German native speaker and user and developer in a English speaking community, I very much like to have an dictionary. It should be offline because to look up simple words I don't need to use the Internet and it is better for privacy.
Until recently, I used KDing (https://en.opensuse.org/KDing) which is fast, has got a fairly good English/German word list and provides an easy way to set a global keyboard shortcut to open the dictionary. Since it is KDE4-based and not maintained, it does not work properly anymore.
Ideally, I would like Plasma to offer easy access to an offline dictionary (read: the best currently available offline dictionary). The "easy" should be so easy that Plasma users do not consider Google Translate anymore for simple translation tasks.