Lacking a common language increases the difficulty of all skill tests involving communication by 1 (for a simple task) or 2 (for a complex task). If the two languages are part of the same group, then these penalties are 0 and +1.
Aquilonian
Corinthian
Nemedian
Ophirean
Zingaran
Brythunian
Hyperborean
Nordheimer
Argossean
Kothic
Shemitish
Zamorian
Zhemri
Darfari
Keshani
Kushite
Punt
Zembabwein
Cimmerian
Hyrkanian
Iranistani
Pictish
Stygian
Turanian
Himelian
Kambujan
Vilayet
Zuagir
Countless dialects and minor tongues
Acheronian
Ancient Stygian
Strong justification is required for the knowledge of these languages. They are, after all, the dead languages of a previous age, not just a previous civilisation.
Atlantean (the continent of Atlantis)
Lemurian (the Lemurian Isles)
Ligurean (history unknown)
Valusian (the continents of Thuria and Mu)