Here's good news.
Masato: Japanese IME "ATOK" (Windows/Mac) allows you to select a term in the source segment and pop up an English-Japanese dictionary
Better than the (free) JP<>EN dictionary that comes with OS X/macOS?
Masato: and that neither can be capable of user dictionaries (if I'm not mistaken).
You are, I'm afraid. Mistaken, that is. In the second screenshot above, I added Technisch, BI>EN, and EN>BI myself, but I might as well have added my own termbases.
My knowledge about Mac seems to be outdated (I don't have a Mac now)!
Masato: I don't have a Mac now
I do feel sorry for you...