A useful metaphor here would be that you are a United Nations (UN) translator and responsible for translating English to Mandarin. If you only have a working understanding of Mandarin, you would not be able to effectively communicate what is being said to the Chinese delegate and what it means in a practical sense. However, if you were raised in the Mandarin province of China and studied communications in an English-speaking country, you can rapidly translate expressions along with important subtleties in the context.