For length in aircraft design, and in weight and balance manuals, what measurement unit does Airbus use?
Structures are soft metric. Airbus uses inch hardware so some measurements make sense in inches but are drawn with an ugly mm number. The coordinate system is in metric.

The rest of the aircraft is hard metric. For example, MTOW and fuel capacity is in a nice number of kg.

Documentation and instrumentation is localized according to the operator. Mass, as in fuel, can be switched between kg and lbs on ECAM, and all the manuals and W&amp;B will follow.

There's a few units that aren't localized, most notably pressure is always in PSI and engine oil is always given in quarts (probably since it comes in quart cans).