Hi Jakub, thanks for taking the effort to simplify AppErrors handling!
The AppErrors.properties file and its localized variants should live in a single location, e.g. frontend/webadmin/modules/frontend. We should avoid having multiple AppErrors.properties copies spread across the codebase.
The backend dal (data access layer) module has its own copy of AppErrors.properties file. One option is to copy it from the source location during Engine build, another option is to simply use symlinks.
In any case, there should be a unit test that ensures all EngineMessage enum members are reflected as methods in AppErrors interface. This will give us the confidence that backend EngineMessage's have proper strings associated with them.
Regards,
Vojtech