I agree that it is better to write in English, the reason I made this question is because many times I have seen long sequences of posts written in other languages (not just spanish, but chinese, german, french, etc.) and I think that is not the right way to help others, because it excludes a lot of people from the conversation.
Maybe a better way would be to take that conversation to a private message until it gets to a significant contribution, and then came back to the main thread in English. Also, if a bilingual sustainer notices that a question has redaction problems because of poor English, she/he may contact the OP in his native language and help him to rephrase the question.
About what you have said, maybe you want to take into account that, in many places in the world with non american or european life standards, there is no easy access to learning English, this I know by experience.