biegły w czymś in inglese:

He is versed in spanish.

The Parisian police, he said, "are exceedingly able in their way. They are persevering, ingenious, cunning, and thoroughly versed in the knowledge which their duties seem chiefly to demand."

Dr. Faust was well versed in alchemy.

That politician is well versed in internal and external conditions.

As may be expected of an expert, he's well versed in the field.

He is well versed in geometry.

The theories of relativity can seem recondite even for people who are well versed in the sciences.