I mean, is human language in flux like everything exists in this world? It's actually argument of Heraclitus which says that everything is in flux or change but Language is fixed system so the faulty and limited but still useful to humans. Is that the really the case?
According to modern linguistics, languages are not static, but rather evolve (see Evolution of languages). Thus, e.g., most languages spoken in Europe today have evolved from a common ancestor (Proto-Indo-European language).
Languages are constantly evolving. New definitions arise from colloquialisms and old definitions fade into disuse. Take the word niggard:
"That niggard never leaves a tip when out for lunch."
This word is fading into extinction due to an unfortunate phonetic coincidence.
Or the word gay: "The party was a gay affair." This sentence had one meaning 100 years ago and it's not the current common meaning ascribed to it.
Dialects, even accents (southern accent vs New England) are constantly changing so I don't believe there is anything static about languages.