What percentage of the English language do you think you know?
Do you think we know more English than just the percentage of the tip of an iceberg?
How much of an iceberg is under water? What percentage?
only a small part of a much larger problem. Only a small, often unrepresentative portion of something much larger or more complex that cannot yet be seen or understood. E.g. This figure represents only the tip of the iceberg, since as many as 90% of cases go unreported. If Congress doesn't vote to extend the debt limit, a government shutdown will be just the tip of the iceberg. The flooding is bad, but we're dealing with just the tip of the iceberg: a huge spate of environmental disasters are on the horizon because of climate change.
I would say we know just the tip of the iceberg about the English language.