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It all depends on the level and subject matter of the text. I'm not very good at literary texts, but I have some knowledge. I'm not good at legal, economic, and other specialized texts. I understand general conversational and everyday language, and I can communicate, but it's still not great; I have to figure things out – I understand intuitively. My grammar is moderately poor. Slang is a complete mystery. Overall, I think I know the language rather poorly. :(
But a dictionary works wonders... :)