(Not a teacher)
Here are a few suggestions:
Sentence 2: Put the article "the" before "law." Can you more precisely state the position of the warning sign? "Before the camera" doesn't quite specify its location-on a pole in front of it, above or below it?
Sentence 3: Put the article "a" before "sign." Change "English-spoken" to "English-speaking." The word "please" probably should be removed.
I don't know if there is a standard warning sign in the U.S. or whether such signs are even required. I believe I have seen "camera-controlled intersection" or "intersection monitored by camera."