All internal calculations performed by Orbiter are done in metric units (metre, second, kilogram, Joule, Pascal, etc.), for the simple reason that this is the only system I am familiar and comfortable with, and it is widely used by the scientific community. Likewise, all standard instrument readouts and data displays are in metric units (with very few exceptions, like the use of astronomical units (AU) for large distances). There is however nothing preventing an addon developer from implementing instruments which display their data in a different unit system, and it would be entirely possible to write imperial unit replacements for all standard MFDs. Just don't expect them to feature in the stock Orbiter distribution. If you want to see feet, fathoms, stones, barns or quarts, you will probably have to code them yourself.