What is it to 'know' a language?

Knowing a language is not a black or white thing. It's not 'either/or'. It's like 50 shades of grey (and then some).

You can never know any language, including your own, completely. Experts might aim for 100%, but they'll never get there. Maybe 99%, or 98% for the rest of us.

Also, I think that it's unrealistic to expect to know another language as well as your first. After all, you've had 24 times 365 times your age hours worth of exposure to English (say). You'll never match that with Thai or Ukrainian.

That said, I'm happy to know a language as well as someone brought up in that language at the point of starting high school. That'd do me. And I'd put the emphasis on understanding it (listening and reading) rather than being able to speak or write it.

Once you understand everything that you read and hear, the rest will come with a bit of practice.