I did some searching when this came up before- basically it can LOOK like they are slower but they aren't. In that they still have say 50 words at 18 months, and are doing 2 word combinations at 2 years for example, but the 50 words is divided between the languages. So in a bilingual household- if you looked at one language only they may appear to have only 25 words, but when you look at both languages they have the full 50. And all kids are different, and that first 2 years they develop so much, and vary so much! Some will be doing sentences earlier, and others will be closer to the norms.
It's important that if you're getting to 18 months or 2 years and you don't have a good bunch of words that you don't just consider that it's a multi language thing, but there may be other things going on. AND generally signing is a great way to support communication and understanding even if there IS anything going on!
