"Piszemy książkę po angielsku."

Translation: We are writing a book in English.

[Tinyset]

Can we say 'We are writing an English book?'

[tadjanow]

I'm not completely sure, but to me 'English book' sounds like a book about English (e.g. a grammar guide) rather than a book written in English.

[fredpareto]

I agree. How would one write 'We are writing an English book' then?

[Emwue]

Assuming it is a book about English, it would be "Piszemy książkę o [języku] angielskim", but I feel most of the times a native speaker(me for sure) would be more specific here, so it would be, for example, "Piszemy książkę o gramatyce angielskiego"(We are writing a book about English grammar), "Piszemy książkę o historii angielskiego(history of English [language]), "[…]o fonologii angielskiego"(English phonology) and so on, depending on the topic.

[wildleoappeared]

That should be correct
It's true that it can have a different meaning but it should be considered a correct translation

[frandogk3]

"An English book" could mean several things, based on context. To be perfectly clear that the book is in English as opposed to someother language, "a book in English" works best.

[JedenPolska]

Does po mean in?

[immery]

only in this context - speaking, writing in a language.

I checked a dictionary http://sjp.pwn.pl/szukaj/po.html po has 19 meanings,

Usually po means after popołudnie= afternoon