Comment by messe

You have an off-by-one error. The strings are 3 bytes long.

I'd write something similar, more or less; Probably the following, not for optimization, but more as a matter of style:

    const char countries[][3] = {
        "A1",
        "A0",
       // [...]
    };

    // [...]

    int total_countries = (int) (sizeof(countries) / sizeof(countries[0]));
    return cc < total_countries ? countries[cc] : "UNKNOWN";

No need to hard code the length, and the cast is guaranteed to be within the bounds of an int on all platforms as long as you don't go over 2^16-1 countries.