The easiest example to find here in South East Asia is for Muslims; the Quran is archaic in that it stays in the Classical Arabic Language, when most other Holy Books get translated to the current Language of their Followers.
Most Muslims know parts of it because they are trained to memorise it regardless what their Native Language is; some can recite the whole Scripture by heart.
So, no; memorising and understanding the Quran does not equate to proficiency in Conversational Arabic, as some would assume.
The second easiest example are Registers.
Higher Registers tend to remain archaic because of their nature and reverence, whereas Lower Registers follow the same path as normal Language and even Colloquialism.
Since my Family is rooted in the Colonial Era, I experience two Registers in my Dutch, i.e. for people older than myself and everyone else, and three in the Javanese Language I use to communicate on the island of Java, i.e. Krama, Madya and Ngoko from Highest to Lowest.
The third would be the Melayu Language usage between Malaysia and Indonesia.
The Melayu People occupy an overlapping area between both Countries and, Historically, their Language is widely used in both, fusing into some of the hundreds of Regional Languages in Indonesia.
Today, it is the Native Language of Malaysia, which basically translates the Country of Malays or the Melayu People, and the Internal Lingua Franca of Indonesia; both Countries use English as the External Lingua Franca.
From a Linguistics standpoint, Malaysians preserve the purest form of Melayu through the centuries, even with Absorptions from various sources; to the Indonesian perspective, the Malaysian Language is archaic.
Understanding both Languages at a Conversational level takes a bit of troubleshooting from either side, especially when considering the nuances of Hokkien in both, but is quite rewarding.
