There are two stipulations when it comes to deciphering ancient script successfully:
1)enough material to work on and the script can be clearly read.
2)a clear link to another known language.
Today, there are some languages which make it hard to decipher even though they are readable. These undecipherable languages generally fall into three categories:
1)an unknown language using known script
2)a known language using unknown script
3)an unknown language using unknown script
It is important to know that many of the unknown, undeciphered scripts have been partially conquered, and linguists continue to work on the remainder. Some examples of partially deciphered scripts/languages include Etruscan, Indus/Harappan, the ancient Mayan scripts of Olmec, Zapotec and Isthmian, Proto-Elamite (the world's oldest undeciphered script), and Rongorongo.
Why some of these scripts present such a challenge:
The Etruscan script was written in an ancient form of Greek, but there is one difference: Etruscan letters point in the opposite direction and are written from right to left. Many attempts have been made to connect Etruscan to known Indo-European languages, but success has largely evaded linguists in this area. There are some 250 agreed upon words, but linguists will continue to come up short until more and better inscriptions are discovered.
2)Mayan(Olmec, Zapotec, Isthmian)
While the Olmecs were first thought to be illiterate, linguists have reversed this hypothesis after an inscribed stone block with 62 symbols was discovered by road builders. However, without any further discovery of other inscriptions, work on deciphering the discovered symbols have largely stalled. This also remains the challenge of the Isthmian script. As for the Zapotecan language group, the scripts are so diversely complex that it is nearly impossible for linguists to reconstruct.
This script is generally found on seals and tablets, but is difficult to decipher because of the lack of complete sentences available for deciphering. The main difficulty lies in the fact that there is no key like the Rosetta stone to connect the same text to other known languages. There is also controversy about the type of script Harappan represents, whether it is alphabetic, syllabic or mono-syllabic.
This script was used 150 years from around 3050 BC in Elam, an area which corresponds to Western Iran today. The interesting thing about Proto-Elamite is that it is preceded by two known scripts, one of them, Linear Elamite. Unfortunately, there is an 800 year gap between each of the three scripts and there are no linguistic bridges between Proto-Elamite and the other two scripts. The difficulty in fully deciphering Proto-Elamite arises from discrepancies between previous and current linguistic interpretations of both scripts: as a Western equivalent, consider that Chaucer or Shakespeare never existed to allow us to connect Anglo-Saxon to modern English. This would have been disastrous.
However, new developments made possible through technological advances have made it possible for Dr. Jacob Dahl, director of the Ancient World Research Cluster to make a breakthrough in deciphering this interesting script. The main difficulties he has faced thus far:
a)the script may have been corrupted; inconsistent patterns in the script have been noted and perhaps, more disturbingly, a lack of scholarly excellence may have contributed to this corruption as well.
b)there is no way of knowing how the words sounded, as Proto-Elamite is not a spoken, but a written language.
c)there are no known bi-lingual texts to explain strange symbols and dashes in the script.
d)although the Proto-Elamites borrowed their language from the Mesopotamians, they also took it upon themselves to invent extra, new symbols.
This Easter Island script is still undeciphered. When missionaries arrived on the island in the 1860's, they were met by Rapanui natives who told the missionaries that wise men with knowledge of the script had all been annihilated by the Peruvians. This presents another difficulty in the deciphering of unknown scripts: when esoteric knowledge is passed on through oral means, it is hard to recover what was once a vibrant language.