Please tell me which one is correct?
please, speak in english.
please, speak english.
thank you in advance
... and so is the second.
thank you everyone
2. Please, speak English.
3. Please speak in English.
4. Please speak English.
It might be worth noting that #1 and #2 have a slightly "imploring" sound; while #3 and #4 might seem brusque.
For an all-purpose polite request, you could say:
5. Could you speak English, please?
6. Would you mind speaking English, please?
Thank you MrPedantic