I can speak both English and Chinese (Mandarin) as a native without any accent. I am definitely more comfortable speaking, reading and writing English, having been living in the States most of my life. I speak Chinese better than 80% of native Chinese because of my Beijing dialect, which is considered as the standard for speaking Chinese. I can't stand it when southern Chinese miss their "h" and "ng" in their words... It just makes my skin crawl... Unfortunately, I can no longer write Chinese, which is a total shame. I can understand Cantonese and Shanghainese if I was given about 10-15 minutes to warm up.
My wife can speak, read and write both English and Chinese (Mandarin) like a native. She also speaks Cantonese like a native (Hong Kong dialect, not Guangzhou dialect). She can also understand Spanish (~50-60%) and some Portuguese (not sure how much).