If everything else is OK, especially hearing, as Humbertbear says, then I shouldn't worry.
In my experience, first-borns like to get it right before they start talking. I remember surprising mine once as he practised words to himself in his room whilst playing with his Lego. Second-borns will just chat away and have a stab at new and difficult words, not caring if they get them wrong. (This pattern can be reversed, but it goes to show that there are different ways of learning to speak)
Several stories come to mind of children who didn't speak for ages, but then started straight away well into their third year with complete sentences. These are probably exaggerated, but it's not impossible....
And as to babies and toddlers understanding more than they can actually say - well, we all do that don't we? We read or hear a word which we can understand from the context, but which we would never use ourselves.
This is called "passive" and "active" vocabulary.
It starts to get really interesting when the children are learning two or three different languages in parallel.