I am an English major and plan to be a translator in a law firm in the future. Is it practical to get a legal English certificate?

Of course, it is meaningful and significant, and it will give you a lot of points, because there is no such thing for English majors in general. You have a certificate in pure English and a certificate that spans two majors, which gives you a huge advantage over others. And since I decided to engage in legal translation, I really need to learn some legal English, which is a necessary condition for my work.