Question

speak in english or speak english , which
is correct?

Answer 1

Hola Estimado !
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Speak in English is Correct !!! Because when you wanna make any person to speak him in English h u should say speak it in English ,Speak English wont applicable as it will only work when you were discussing in sitting with more members 

Answer 2

Speak in English is correct.

for example:
we generally say"do you speak in english?"

Again,
we say" please can u speak in English? "

hope it helps....