It would vary! Jersey's population has been a mix of English and French families for a long time. And people in St Helier would be more likely to be English-speaking while people in the rural areas were more likely to be French-speaking.
If you had, say, an English soldier marrying an English-speaking girl from St Helier, the record would likely be in English. But if a Frenchman who'd come to the island to work married a French-speaking girl from a rural parish, it'd be in French. I have both scenarios among my own ancestors!