That depends a lot on the situation, and our interpretation of what you asked.
My interpretation of your question is that you are referring to a situation where someone is speaking English - say in a store in China, and you would like to speak to them, then that would again depend on the situation. If perhaps they were struggling to understand, you could perhaps say 'I speak some English, could I help you at all?'. If they were not struggling, you could perhaps make some comment on what they are looking at, e.g. 'I think that shirt looks nice on you'. Say something that is not too invasive, so they can choose to follow up your comment, or not. Obviously if they seem in a hurry, then perhaps leave it until another time.
That is more of a social question than a question of language - it would be little different to saying something to a stranger in Chinese.
If you meant to say 'someone who speaks English', such as here on italki, then that is quite different. We are all here because of an interest in language, so it would be perfectly fine to just introduce yourself, and offer a conversation.
If you can be a bit more specific about the sort of situation you are thinking about, we can perhaps offer more concrete suggestions.