I think it is an open question: can an unknown language be cracked -- without any dictionary or grammar or understanding of the language? Just lots and lots of texts, maybe some of it bilingual.

It's a common misconception that is what happened with Ancient Egyptian with the Rosetta Stone. The Rosetta Stone was just one of the big pieces of the puzzle. The decoding came when people realized that Coptic (a language written alphabetically and still in use in the Coptic Church today) is actually descended from Ancient Egyptian; as Spanish is to Latin, Coptic is to Ancient Egyptian.

Similarly the attempts to decode classical Maya were all dead ends. Until Yuri Knorozov realized that it encoded the ancestor of the Maya languages which are still spoken to this day. (Knorozov's Wikipedia article is worth checking out just for his photo with his cat. [0] IMHO.)

I have written before about the La Mojarra 1 stele in Mexico [1]. It looks a lot like Maya. [2] But it isn't Maya. Maybe the difference like between Russian and Latin writing?

No one can read it. It's undecipherable. There are some attempts to identify it with a proposed ancient language that would have been related to the modern Mixe-Zoque languages: some of the glyphs that are shared with Maya, when read phonetically, start sounding like a Mixe-Zoque language. But no one has proposed a confident decipherment. There probably isn't enough text. La Mojarra 1 is the only long example of the Isthmian script.

Deciphering Akkadian was very difficult, at first. The process started with Persian; old Persian was written in a simplified adapted form of the Mesopotamian cuneiform (wedges on clay). It was a kind of alphabet. And Old Persian was already understood. And there was a bilingual text on a monument carved by Darius I. But even then -- decoding relies so heavily on the fact that Akkadian is a Semitic language distantly related to Hebrew, more distantly, also Ancient Egyptian. So again, we sort of knew what we were looking for.

That is all to say: even if the Voynich manuscript (for example) contains real text in an otherwise completely lost language, I'm not sure it is possible even theoretically to translate it.

[0] https://en.wikipedia.org/wiki/Yuri_Knorozov

[1] https://en.wikipedia.org/wiki/La_Mojarra_Stela_1

[2] https://commons.wikimedia.org/wiki/File:La_Mojarra_Stela_1_S...

Amateur may have cracked Linear A, a 120-year-old puzzle