This is your skill with the dead language of a dead culture. At the higher levels, you have an appreciation of linguistics and can track the evolution of languages from your studied root language. You understand the idiomata and how it relates to the rituals of that culture when combined with Archaeology. Although you must choose an ancient language as your most studied, you may be crudely familiar with other related ancient languages in general.
In ancient times, these civilizations are not so buried in the past. How much you could possibly know, given the primitive standards of the setting, is quite limited and possibly based on myth. In the future, the skill would be aided and tracked using computers; it would be best to combine this skill with a good background in Technology.