For writing samples, is it necessary to provide an English translation of a conference paper written and presented in my native language?
It would be advisable to submit a translated copy of the paper (with the original) for a PhD program in the US (or any English speaking country), as chances are, the people reviewing your application may not speak your language nor have easy access, or time to have the document translated.

It may be wise to submit the original with the translation, with an official note stating that it is a true and accurate translation (say from your university or the publisher if they provide this service).