This question is pretty complex to answer, because even modern Chinese linguists cannot reach an agreement on how to explain the title of Fangyan.
Now, I'd pick explanation from Wang Li(王力) and He Jiuying(何九盈), but note there're also other explanations.
輶軒使者:
- 輶軒: A kind of lightweight vehicles.
- 使者: Officers sent by emperors to local places.
- 輶軒使者: A special kind of officers sent by emperors in Zhou and Qin, who took youxuan(輶軒) vehicles to other places and recorded local dialects.
絕代語釋:
- 絕代: (adjective) Something no longer exists(絕) in this era(代).
- 絕代語: Speech no longer exists in this era.
- 絕代語釋: Explanations to the speech no longer exists in this era.
輶軒使者絕代語釋: Explanations from youxuan officers to the speech no longer exists in this era.
About 輶軒使者: Now we know that 輶軒使者 is a proper noun refers to those officers sent by emperors of Qin and Zhou to record dialects. These officers are called youxuan(輶軒) officers because their main feature is that they took lightweight vehicles called youxuan(輶軒) to go to local areas. Take this in mind, even if you want to translate this word 輶軒使者, I'd recommend just translate it to something like "Youxuan Officers".
About 絕代語釋: The author Yang Xiong(揚雄) lived in Han Dynasty, to whom the materials from Youxuan Officers were historical materials, and their explanations to dialects at that time(Qin and Zhou) had become "絕代語釋"(explanations to the speeches no longer exists in this era).
Therefore, the title in fact consists of two parts: 輶軒使者絕代語釋 and 別國方言(That's why Tangshu(《唐書》) abbreviated it to 《別國方言》 while Chen Zhensun(陳振孫) abbreviated it to 《輶軒使者絕代語》). If you'd like translate it, it should be something like:
Legacy Explanations to Ancient Speech from Youxuan Officers and Explanations to Regional Speech
It is roughly like:
Dictionary of Historical and Dialect Words
It make sense cause historical words can be maintained in some dialects but disappear in others. These words are dialect words synchronically and also historical words diachronically. Even more complicated, they had been dialect words historically but the distributions of them are different now.