The reason there’s so much Latin in the law is that the present-day legal systems of the US, UK, Canada, Australia, and much of Europe are ultimately direct or indirect heirs to the Roman legal system, and to the attendant language and terminology imposed on its subjects during Rome’s conquest.
Over 1500 years have passed since the fall of the Roman Empire, and yet here we are as a profession, continuing to use terminology like sui generis, habeas corpus and mutatis mutandis. (A brief and fascinating explication of this can be found here.)
This is generally understood among us. But what’s with all the French in the law of the common law countries? Force majeure or voir dire, anyone? Even today, the hail that court clerks in the US and England call when the judge enters the courtroom is French in origin: oyez (citation).