While I know the word kindling in English (which is a second language for me), I don't even know it in my native language... So, I don't think that's a good tes

While I know the word kindling in English (which is a second language for me), I don't even know it in my native language... So, I don't think that's a good test. I'd say that someone is fluent when he starts thinking in the language and is able to have everyday conversations with other people at normal speed without the native interlocutors feeling that they need to simplify their speech.