Why a creole is handy in Germania

Caesar states that there are Celtic tribes beyond, east of the
Germanic tribes. That strengthens one's suspicion that the Germanic
languages are a recent introduction to "Germania".

Caesar's campaigns and conquests in Gallia must have caused uproar
among the then Celtic-speaking tribes east of the Rhine, in what
later would be known as Germania. Would it be wrong to assume
that "Odin"(Ariovist?) played the a similar role east of the Rhine as
Vercingetorix west of it (except of course that he spoke a foreign
langage and that he was ultimately successful, in which respects he
was more like Caesar)?

Given the tendency of the Celtic languages, locally under the control
of druids, to fragment, how much more difficult would it not have
been for the Gallic peoples to coordinate their efforts under
Vercincetorix at Alesia, than it was for the (newly) Germanic peoples
speaking among themselves in the new (and druid-less!) creole
language to do the same under Arminius when facing Varrus' legions?

We all try to understand past times and unfamiliar cultures in terms
of what we know. Here's my angle:

Among the bourgoisie in Scandinavia there has been since the 19th
century a Scandinavistic movement. People studied each other's
languages and declared that it was easy for Scandinavians to
understand each other's language. Which is true, in a sense, with a
caveat:

Words that are current in one Scandinavian language mostly are found
in the other ones too, but in some cases as poetic, obsolescent or
obsolete words. Example: Danish {aften} "evening", Swedish (poetic,
obsolecent) {afton} the same; Swedish {kväll} "evening", Danish
(poetic, obsolescent) {kvæld} the same. Thus, paradoxically, the
larger your vocabulary in your own language the greater your ability
to understand the other Scandinavian languages. And, the inverse is
also true.

This means that with the wide-spread use of English (and the
consequent(?) reduced knowledge of their own language) people will
resort to using English when communicating with other Scandinavians.
Knowing and understanding the other Scandinavian languages is
considered nerdy.

I imagine a similar situation during Caesar's campaigns in Gaul:
Vergingetorix and other -rix'es and druids meet and communicate in
various Gallic dialects, understanding each other (having a good
druidic upbringing) and feeling very Gallic. But at Alesia
communication among the ordinary Gallic people breaks down, plenty of
misunderstandings. "Celticism" ("Gallism"?) to them has become an
intellectual abstraction. Why die for the druids?

I imagine Pan-Slavism has had a similar fate?

Any dictatorial nation-builder will want to enforce if not ethnic,
then at least linguistic unity. Any group speaking a foreign tongue
within the empire are potential traitors: who knows what they are
saying in their gibberish code? One powerful method is to use
deportations; perhaps this is how we should see the arrival of 28000
Charudes from Jutland to Ariovist for resettlement?

Torsten