Certainly, ethnogenesis is a difficult subject.
We cannot be sure that Hallstatt people were speaking the exactly same language as La Tene one, as we have not found written proofs. But I don’t know many present historians doubting that Hallstatt people were celtic speakers.
About scientist assuming that the celtic language was in use in western Europe far before the emergence of the Hallstatt culture, I would quote some of these :
- Venceslas KRUTA ( Les Celtes – 2000) :
“It appears more and more clearly that … the extension of celtic languages cannot be considered as the result of a late extension from the center-european core.”
- Patrice BRUN ( article from “La préhistoire des Celtes” 2005 – Celtes prehistory)
He assumes the emergence of celtic languages within the 3rd millennium BC.
- John COLLIS (The Celts – Origins, myths inventions – 2003)
“There are two distinct schools of thoughts about the origin and spread of the indo-European languages :
- The one of Colin Renfrew who links it with the spread of early farmers.
- The alternative is to make the time-scale much shorter, from the early Bronze Age at the earliest…”
- Pierre Yves MILCENT (archeologist)
More generally, the Hallstatt – La Tene model, assuming that all celtic speakers came from central Europe is no longer assumed as the wright model. Too many archeological proofs demonstrate that other celtic cultures were existing, before or at the same time, and elsewhere.
One of the latest contribution to that is “Celtic from the west - 2010” by Barry Cunliffe and John Koch.
Obviously, as the old model is no longer asserted, there is not yet a new model asserted by a majority of historians !