Asklemmy

It's worth mentioning that the word bilingual has different meanings in US English and in British English.

For native British speakers, someone who is bilingual is someone who speaks two languages at a native level, while the accepted US meaning is someone who can speak two languages, especially with equal or nearly equal fluency.

With the British definition, it's pretty clear whether someone is bilingual or not. Most people are not, and it's almost impossible for an adult to become bilingual later in life. Generally it only happens when someone has two parents each with a different mother tongue.

The US meaning is much wider than the British one, and I guess it's the meaning you're intending with your question. It basically comes down to the definition of fluent. It's completely possible to be fluent in a language while still having a foreign accent and still making the occasional grammar mistake. My personal definition of fluency is when you are able to talk to native speakers on pretty much any subject without serious misunderstandings. You don't need to know every word you may encounter, as you can simply ask the other person what a word means just as native speakers do all the time.