my ds is 15mo and has only just started in last couple of days saying mummy or mama. before that nothing - apart from a brief go at the dog's name, which he did for a bit and then gave up on. the other babies of the same age in our NCT group are exactly the same. my nephew didnt say anything at all til about 19 months and now doesnt shut up!
so she sounds v normal. and i bet she does understand a lot. try asking her to give you something that's nearby that she;s familiar with, like a teddy, and see if she picks the right thing up. eg ds, if you say "shall we read a book?" will usually go and get a book, but couldn't say "book" to save his life
if you want to speed thngs up the advice my sister was given about my nephew was talk to him continuously, ensure you are saying things really slowly and clearly, and repeat the same word a lot in slightly different phrases (eg if he's in the pushchair and a cat goes across the pavement, you say "look, a cat!" and then you say 'can you see the cat?' and then 'oh, it's a black cat'). supposedly that reinforces that the common word is 'cat' ie that's the name for what you can see. also describe the things she's doing while she's doing them eg 'you're playing with your ball. can you roll the ball? now you're throwing the ball at me in exasperation because i keep talking at you.'
or whatever.
and yes, you do feel like a total idiot doing it especially in public. but it can't hurt....