Guys, please... I hope 99% of this thread is not serious despite the original poster actually asking to be enlightened, we should enlighten him, not confuse him more.
I am no expert on Celts, but on Germans and Rome (Tacitus mentions, BTW, that the Germans bathed in cold rivers when they got up- but he may just be passing stereotypical stuff). In any case I know what the Romans had that most others did not.
However, let us first define "barbarian" It has passed to us from Latin, which again had it from greek βάρβαρος (bárbaros). This basically mean a non-Greek and imitates the weird sounds they made when speaking instead of using a civilised language (Greek). The Athenians even used it to describe and deride other Greek tribes/polis on occasion. Though Plato rejected its use at all as it told nothing of the barbarians.
In any case, to the Greeks, then Romans it meant a person that was not Greek (or later Roman, for in fact the Romans were barbarians to the civilised Greeks, at least until they were conquered and their culture conquered the more brute and primitive Roman one). However it came to mean a pejorative term for an uncivilized person, either in a general reference to a member of a nation or ethnos, typically a tribal society as seen by an urban civilization either viewed as inferior, or admired as a noble savage (Tacitus on germans for example). There are similar ideas/labels in all urban societies across the world. I guess the Greeks were barbarians to the first civilised urban societies in "The Fertile Crest" where civilisation rose too.
Anyway, now for why the Celts were seen as barbarians by the Romans.
Actually go look here and have a laugh
https://www.youtube.com/watch?v=ExWfh6sGyso the Monthy Python crew were not unenlightened...
Rome had an organised state and army, a constitution, freedom (no kings) a well developed parlamentary system that was a model for at least one major modern one.
Rome had paved roads that endures to this day, bridges, acqueducts and sewers, temples, forums and arenas. Not to mention the theaters and the medicine. And while we are at it, Rome had science, it had engineering and medicine indeed, it had learned scholars, mathematicians and philosophers.
Now, much of this was learned from Greeks (and others mainly Etruscans), but Rome had it all, and Rome's genius was in adapting, learning from others and in fact not be xenophobic (Roman citizenship was always open to people of merit even in the Res Publica Romana, some argue that this strenghtened the realm others that when they diluted what was Roman by becoming multiethnic Rome became weak and fell).
You also seem to forget that Rome was not = Romans even when it was but a city-state in Latium. Most Romans were farmers, all soldiers came from peasant stock as these were hardy, strong and had many skills already that was required for life in the army as well as better suited to subject themselves to discipline and hardship then soft and spoiled city-dwellers. The point being that earth-grubbers are earth-grubbers everywhere. A Roman peasant would have been as dirty as a Celtic one, and both would be careful to wash a bit before eating and such. Being barbarian is not defined by your cleanliness, it is if anything defined by being part of an urban civilisation.
Anyway I suspect the above are the reasons that Romans considered Celts barbarians. Despite the neo-romantic Celtic drivel idealising them as a high culture, they were
not.
I hope this helped you Caligula (may I ask why on earth you chose that name? You might as well call yourself Idi Amin

) and I both welcome you and remind you that by installing EB you have agreed to read more history. I definately encourage that;-)
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