The Baby, when can it start to talk?

Well, my daughter has recently made a baby, it is 6months old but i think it may be a bit 'slow' if you know what i mean. Sometime it just cry and cry and when i try and hit it gently it on bottom it still cry and cry. Sometime i sing it indigenous lullaby to no avail! It still wear the nappy and cannot really talk to me. My daughter say it perfectly normal and healthy child but is she hiding some secret from me?