Earlier quoted context omitted.
I think people (here and below) are getting hung up on definite articles, but Zipf's Law makes no such observation. It says only that a word's frequency in a natural language corpus tends to be in inverse proportion to its rank in a frequency table. In English, the most frequent words are articles, but the general observation about word frequency holds across languages (whether those languages have articles or not).
"The most frequently appearing words in this pile of un-translateable text are the most common words in the language it is written in" seems like it falls somewhere between blindingly obvious, and entirely useless. Unless you have some clue what those words mean, how does that observation help you?
It doesn't tell you what those words are, but it is a pretty specific observation about the frequency/rank relationship. So, as the wikipedia article liked about points out, it can tell us that the Voynich Manuscript was probably written in a language (of course, it could be a cypher of a real language or something made up, like elvish in Lord of the Rings, but it probably isn't just a random collection of symbols because it is unlikely that a random collection of symbols would happen to follow this distribution).