How do engineers log airplane maintenance? Is there an encoding system that replaces the vagueness of natural language when it comes to logging aircraft problems or repairs that took place at a specific site on the airplane? 

So, instead of someone having to enter a description like:
"Hydraulic fluid change at system ABC"
Which would be in their own native language and filtered by their own personal level of awareness, they could enter something like:
"GF65Z ABC"

Therefore, internationally, GF65Z could correspond to exactly that type of intervention with some hydraulic system and that would also make further investigation, in the case of some problem, easier.

Anywhere I can find more information about something like this?
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.