That is a very good question.
There were either a Germanic speakers tribe, but I have no idea what branch of Germanic they were speaking, if it was West or East,less likely Norse.
They could have been also part of the large Gauls/Keltoi ethnicity which had different languages.
I do not know how early Gauls/Keltoi had their languages.
Cimmerians could have also been Thracian speakers.
Or they could have been Iranian speakers.
What a multicultural nation they were! It can be true that they lived in different Germanic, Celtic and Thracian lands but they were an Iranian-speaking people, of course Persian (Cimmerian) is actually a Thraco-Iranian language, mostly because different sound changes in this language, for example we see *
ḱ>s in Indo-Iranian but
*k>θ in Thraco-Iranian or *
ǵ>z in Indo-Iranian but *
ǵ>ð in Thraco-Iranian, ... compare Old Persian
daθa "ten" but Indo-Iranian
dasa, Greek
déka, ...
I think you are a Romanian, ancient Persians considered the northern part of Romania as one of their original lands, it is interesting to read it:
http://www.livius.org/articles/person/darius-the-great/sources/the-gherla-inscription/
One of the most remarkable inscriptions from Antiquity is the following fragmentary text, written in Persian cuneiform script, found in 1937 on a clay tablet in Gherla, and published in 1954:
[...] king [Darius] son of Hystaspes [...]
[...] did [...]
This is of course not a text full of very important information, but the fact that it exists is remarkable, because Gherla is in the northwestern part of modern Romania.
Although this is the only Achaemenid royal inscription that was ever found in Europe, it is not without parallel. The Greek researcher Herodotus of Halicarnassus tells us that Darius, after conquering eastern Thrace (in c.514), visited the source of a river and left the following inscription:
"The head-springs of the Tearus give the best and fairest water of all rivers; and to them came (leading an army against the Scythians) the best and fairest of all men, Darius, son of Hystaspes, of the Persians and of all the continent king."