I would say that it depends on whether that language would have wanted to be decyphered.
The origin of Cryptography dates back to around 700-800 AD.
We may not know the methods by which these texts are obscurred. One such example is the Lesser Banishing Ritual of the Pentagram which, revealed only by happenstance, formed the basis of a whole new esoteric renaissance in the west, the lexicon included forming a basis for the lexicon in use by these orders today.
This Rite centers around the letters INRI, which tradition says were written upon the cross of Jesus Christ as an abbreviation for Jesus of Nazareth, King of the Jews. There are, however, numerous other levels of occult meaning regarding these four letters in the Rosicrucian Magical Tradition. One of these is a Hermetic secret alluded to by the Latin phrase "Igne Natura Renovatur Integra" which means "By fire, nature is perfectly renewed." These four letters additionally adorn the rays of the angles of the Rose Cross Lamen worn by Adepts of the Ordo Rosae Rubeae et Aureae Crucis.
A deeper interpretation lies occulted behind the attributions of the Hebrew letters and the Magical Forces to the Paths on the Qabalistic Tree of Life. The Path attributed to the Hebrew letter y is attributed to the Zodiacal Sign Virgo as well, that of n to Scorpio, and r to the Sun. There exist further magical associations between the Sign Virgo with the Egyptian Goddess Isis, Scorpio with Apophis, and the Sun with Osiris. When the first letter is taken from the Names of each of these Gods, the name "IAO" is formed. Additionally, due to the Signs associated with Isis, Apophis, and Osiris, they form the letters "LVX."
Thus within the letters IRNI lie concealed the letters IAO and LVX, which may also be found upon the rays of the angles of the Rose Cross Lamen worn by Adepts of the R. R. et A. C. The name IAO was considered by the Gnostics to be the Supreme Name of God. Its letters further allude to Salt, Sulfur, and Mercury in Alchemy and to an even more recondite secret symbolized by the relationship between Isis, Apophis, and Osiris.
So I would say for machine learning to be able to decipher real languages, it would almost have to be intuitive, able to draw parallels and connect various points of reference together. It would have to determine what things mean, when a lot of the times, this is virtually impossible.
Your variables are infinite. You have no real method of finding out what they are. Add to that context, lexicons, deliberate obfuscation, culture and the passage of time and I think it would not be realistic to expect machine-learning to provide much help.