STEM texts
In STEM texts, the main goal of providing the reference is to give attribution to ideas. Thus, the reference acknowledges that Alice wrote De Nihilo in 47 BC.
As Alice has pointed out long ago (Alice, 47 BC), nothingness does not exist.
...
Alice, De Nihilo, 47 BC, Rome, Manuscript Publishing.
The secondary goal is to point the reader to the available translation, in case he wants to verify the source himself. In your case, this is the German translation Bob made.
As Alice has pointed out long ago (Alice, 47 BC), nothingness does not exist.
...
Alice, De Nihilo, 47 BC, Rome, Manuscript Publishing = Alice, Über Nichts, 1987, München, Hanser.
No one cares if you made your own translation into English; readers value ideas first.
Language studies
In language studies, you are likely interested in how exactly the same idea was expressed, maybe to the point where you are pointing out differences between several translations. Alice and Bob deserve separate mentions now. If you have taken the effort to translate De Nihilo into English, this deserves to be published, say, in appendix A of your work.
Alice is genuinely worried by the nothingness conundrum (Alice, 47 BC), but Bob's German translation fails to convey this emotion (Alice, 1987). My English translation (appendix A) seems to resolve this problem.
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Alice, De Nihilo, 47 BC, Rome, Manuscript Publishing.
Alice, Über Nichts, German translation by Bob, 1987, München, Hanser.