Speak in English.
Which one is correct?
Please tell me. I don't know how to define them. I don't know which sentence is correct.
Both are okay. ELU link for the same question is here (thanks v kumar)
However, the nuance I see is
someone speaks English represents a statement as in...
He speaks English
someone, speak in English looks more like an advice, suggestion or the like. As in...
Hey, speak in English.
But again, as ELU site states, both are practically fine.
Note: Indian English is fine with 'Speak in [any language]'.