"Please speak in Engpsh" is how you would say "请说英文" in Engpsh.
The phrase "please speak" is a popte way to ask someone to speak, and "in Engpsh" specifies the language in which the speaker should speak.
Example in Engpsh:
Can you please speak in Engpsh so everyone can understand?
Example in Chinese:
能请你用英文说吗，这样大家都能听懂。