What concerns the speaking skills, being able to communicate with natives flawlessly and comprehend their expressions ,jokes and slang even, when meeting them in their own country, where this language is spoken. As for the written skills one should be able to write impeccably comprehensible articles, letters or any form of formal correspondence. Being able to read novels and comprehend their context in detail is another sign that you are mastering that foreign language. And you are right about simultaneous translation; it could also be a good indicator of being really proficient in a foreign language.

It means that you are able to speak, read and write fluently and without hesitation like a native speaker.