The Nosey Genealogist has an interesting recent set of posts on how to start researching family history in Jersey, and provides a potted introduction to the language situation:
One of the first things you are going to find, if you are researching your ancestors from Jersey is that the records are invariably going to be in French, as this was the official language of this island until very recently when English has become dominant. James pointed out that Jersey was very largely French or Jerriais-speaking until the middle of the 19th century, and so a lot of legal records long after that were kept in French. The deeds to my house, for example.
I have often heard people in the island refer to these documents being written in “proper French” to distinguish the language used from Jerriais, the name given to the Jersey French patois spoken in the island, which even comes with variations in pronunciation across the 45 square miles of the island!