Our son is also 16 months and I was starting to worry slightly as he didn't seem to be saying any words until recently. He didn't even say Mama or Papa. I had read that at one year, children should be saying one or two words in addition to Mummy and Daddy. However, in recent weeks, we suddenly started to understand a lot of words from my son.
He currently only uses short, one syllable words and they're not very clear which makes me think that perhaps he was saying them sooner than we realised, we just didn't understand. I think we were also a bit strict with what we counted as a word but after reading up a bit, I found several sources that reassured me that the word doesn't need to be pronounced perfectly to count. So I started to count a few words that I didn't consider a proper word before. For example my son says dog without pronouncing the g but I count it because he says it in the correct context. I also started to count some things like animal noises.
I found these two questions and answers useful and reassuring:
How important is the pronunciation of a toddler's first words?
How to know if 12 month old is talking
Just prior to understanding some of his words, it started to become clear that our son was understanding more and more of what we said. Since your son understands what you say, I imagine that he will soon start saying some words. With our son, he went quickly from saying nothing which we counted as a word to now saying somewhere around 20-30 words. The first words we understood were things he was interested in like his shoes, dog, cat, duck. I would suggest listening carefully when he's excited about something he likes a lot and you may start to understand something that he's trying to say. Don't be too strict on the definition of a word.
We speak two languages to our son which as you say is generally accepted to cause later speaking and you are speaking three so I imagine that effect is multiplied. In addition to understanding what he's trying to say, you'll have to try and figure out which language it is.