Because of their length and information density IPv6 addresses are unpopular compared to IPv4. They are a lot harder to exchange via text or spoken word because of their length. A single mistake will make the connection fail but without any feedback that it's the fault of the address exchange.
BIP39 is a way of converting bits of entropy to mnemonic sentences, which are easier to speak out and remember than hexadecimal numbers. While it's commonly used for keys in cryptography, it may prove useful for exchanging IPv6 addresses as well.
IPv6 uses a 128 bits address. For 128 bits BIP39 adds a checksum of 4 bits and generates a mnemonic sentence of 12 words.
BIP39 supports custom word lists, allowing to support multiple languages. This would make it easier for the speakers of a specific language to verbally exchange addresses.