Because the language strings are hidden inside a function, you add code that looks at a locale setting to switch language without the caller needing to know.
for that purpose they need not be 'hidden' only "declared only accessible through function call" rather than "accessible directly"

And you can make use of this function from many places, while always knowing that there is a range check performed. Directly accessing the array means you are distributing the responsibility for such things into an unknown number of places.
Not arguing that part; however, when discussed in a '51 forum the array should be 'code'.

Encapsulation really is one of the cornerstones when discussing software design
not really more like "Encapsulation really is one of the cornerstones when discussing software where you are protected against yourself and others working on the project."

I can, to a large extent, agree with your points when discussing projects larger than you see implemented in a '51
e.g. in the current 512k ARM project with about 7 people working on ir there is much encapsulation (some done by me).

Erik