When she turned 2, I had concerns. There is
a difference between
speech and receptive language. When there is
a lack
of receptive language (i.e. understanding directions, responding to their name, etc), then there is usually more concern. I didn't know there was a difference before my daughter turned 2. At 17 months, I wouldn't worry too much.
Some things speech therapists do for kids with just speech delays, is repeat a word lots of times.