I’ve been a full-time translator in Taiwan for the past 16 years, and have done some copywriting as well. There’s a large market for translation, both English to Chinese and Chinese to English, but it’s very disorganized. There currently is no certification or accreditation system for translators in Taiwan (other than academic degrees in the translation field). There’s also a market for copywriting, but it’s smaller and harder to break into than translation (though the two markets overlap to some extent, as many clients seeking “translation” are actually more in need of copywriting.)
If you want to break into the translation field, I suggest you start by trying to pick up assignments at an agency. Pristine would be a good place to start (a Google search will turn up their contact info). Even if you have perfect bilingual skills, you will still have to hone some translation skills, and an agency can like Pristine has experienced translators who can help with that. (I’m not associated with them at present, but worked for them for a while many years ago.)
I also suggest you search the term “translation” on Forumosa. Of particular value should be any comments by Ironlady, the moderator of the Learning Chinese board – she is also an experienced translator.