I think German origin is also connected with the common use of Germany and German to indicate antagonists in fiction. I think a few antagonists in earlier Tales games has some German-named artes, but not a single protagonist I think. Well, since it's mostly speculation I decided to leave that out.—Kaimi (999,999 CP/5 TP) ∙ 23:58, April 4, 2017 (UTC)
I guess Nachtigal derives more from "Nachtigall" which is the german word for "Nightingale". As far as I know (if the information from Taleslation are right) he, his father and his brothers are all named after birds (Milan = Milvus, Adler = Eagle, Falco = "Falke" = Hawk).
Of course since it is still a german origin it does not change anything for your speculation that he is called that because he is an antagonist. :) I think that german really is more connected with antagonists than with protagonists in fictions.
This information can replace the existing trivia, as I do hold Taleslations as a valid source.