* Number of speakers: This is a common metric, but it's not always accurate, as some languages have very small, isolated communities that are difficult to count.
* Geographical distribution: A language spoken by a small number of people in a single, isolated location might be considered rarer than a language spoken by a larger number of people spread across multiple regions.
* Vitality: A language that is only spoken by the elderly and not being passed down to younger generations is considered less vital than a language that is still being learned by children.
That being said, some candidates for the rarest spoken language include:
* Ainu: Spoken by a handful of elderly people on Hokkaido Island in Japan.
* Burushaski: Spoken by around 30,000 people in the Hunza Valley in Pakistan.
* Tauya: Spoken by an estimated 300 people in a remote part of Papua New Guinea.
* Arem: Spoken by about 100 people in a village in Ethiopia.
It's important to note that the number of speakers of these languages is constantly changing, and new discoveries are being made all the time.
The preservation of rare languages is a critical issue, as they represent a unique cultural heritage and are invaluable to understanding the history and diversity of human language.