I think that's the best solution (the only potential optimization I can think of would be to add a padding byte after every country to force 32bit alignment which would make the offset computation faster on some architectures, although probably not on x86).

Stylistically-wise I think the best solution would be to write the array like:

    const char countries[][3] = {
        [0] = "A1",
        [1] = "A0",
    };

This way the codes are explicit when you read the code and it makes editing the array a little easier. Unfortunately gcc only warns if you set the same index twice with -Wextra, it remains silent with -Wall.