That would assume that language is a driving force of culture, rather than its reflection.
I'm thinking of language being a reflection of culture, as you are saying. Then, if the culture changes rapidly, maybe language (which is optimised for the previous culture) will no longer be optimal for reflecting the new culture. I'm not sure that could really happen, I was just speculating!
As well as culture driving language, maybe language can also influence culture? If we accept Chomsky's Universal grammar, there are limits on the forms that human language can take, and hence maybe also limits on human thought and culture. Similarly, maybe specific languages place limits on the cultures they are part of, by making certain ideas difficult or impossible to articulate? And maybe these limits vary from language to language?
IMO, ideas are born inside our brains, not on the tips of our tongues.
I would agree, but language is in our brains, too, so it wouldn't be too surprising if the two influence each other.
Or perhaps I am meandering around a wrong idea of what you mean. Care to offer examples of "external agent"?
Or perhaps I'm just speculating a little too freely! I'm thinking of some of the peoples of the Amazon Rainforest, who are suddenly exposed to modernity. This can produce such rapid and dislocating changes in their society that maybe their language cannot change quickly enough to optimally express all that they now need to express?