No, the article is not necessary here. According to me, it sounds more natural without an article. :-)
Is "ddim" in Welsh related to "δεν"? It sounds quite similar
No; ddim is soft mutated from dim "thing", and δεν is from older ουδέ as far as I know.
I'm not saying 'dim' (thing). I meant 'ddim' (not) as in "Dw i ddim yn siarad Cymraeg" for negation. By the way, does 'dim' really mean 'thing'?
http://www.geiriadur.ac.uk/gpc/gpc.html?dim : thing, something, anything, any matter, aught; any part or quantity, least thing or particle.
GPC is partly a historical dictionary and some of the meanings are a bit old, but that is or at least was one meaning of the word.
ddim is the soft-mutated form of dim.
I don't know the historical linguistics, but I think it arose in a similar way to the French ne ... pas which was originally "not a step", I think, and then got weakened to simply "not" -- thus also in Welsh, something like nid gwelodd ef ddim "he did not see (a) thing" turned into gwelodd e ddim "he did not see", and if you want to say "he did not see (a single) thing" in modern Welsh you'd usually need gwelodd e ddim byd, by my understanding.
At any rate, ddim is not a separate word from dim in terms of word origin / etymology.
Thanks for clarifying! Diolch! Merci!
No, it doesn't -- just uses negative + "to be" as in English.