How do engineers log airplane maintenance?
Natural language is concrete enough for this purpose, the logs aren't meant for machine parsing but instead for pilots, inspectors and other mechanics.

Having such a code system requires mechanics to memorize all of the codes or have a massive list handy when filling out the logs. 

Also making such an international code system requires agencies of all countries/manufacturers to sit together and set those codes for all possible maintenance actions. There is a very high chance an action will be missed so a natural language box for "additional comments" is still required. It also needs to be rolled out to the actual mechanics filling out the logs.