"The engineer speaks only English" is fine, actually. The placenent of "only" is tricky in English. In everyday discourse, we usually place it as suggested by Nanami, "The engineer only speaks English." But here the word "only" qualifies or limits "speaks" so the sense of the sentence could be "The engineer only SPEAKS English [i.e.: she can't read English or write English, but she can speak it]." Whereas when we say that the engineer "speaks only English" or that she "speaks English only" it is clear that we mean English is her only spoken language.