Language in the CG
In past eras, the different peoples of the CG spoke different languages: the Sturians spoke Sturian, the Kislennic tribes spoke Old Kislennic, the Gaeldic tribes spoke Old Gaeldic, and so on. The mahiri spoke Ancient Mahiri, while the dwarves spoke a language now called Middle Dwarven.
Over the centuries, as these cultures came into contact with each other, they began to exchange words and phrases from one another, and they evolved toward one another. Nowadays, everyone speaks a mutually intelligible speech; the dwarves' "Low Dwarven" is basically the same as the speech of the Shengans.
The ancient languages of Dosan underwent a similar merging. When the Yamamotoans arrived in Hesket, they learned that the Hesketines and Dosanese and Hesketines even speak the same language nowadays. (Scholars have a variety of theories as to why that might be.)
Each people or region may still have its own dialect reflecting words from the older tongues, which is why a Chardrois says "Bon jour" but a Kislevian "Guten Tag" or a Zdunarovan "Privet". It's also why a Kjoln nobleman is a Jarl instead of an Earl. Different areas have different degrees of dialect.
Since the Collegium is in Tolbury, the dialect of theology and diplomacy is often that spoken in Allondell. Thus, "proper" speech is often "Allondine" instead of the local dialect.
Archaic English words were also more common in previous eras of the CG. For instance, a writer in the CG's Dark Ages might say "thee" and "thou" instead of "you". The modern speech of the CG uses "you", like modern English does.
Using the Ancient Languages
See the rules on how to use ancient languages in-game.
There is also information on how past GMs have represented these languages.