It sounds like something that my 2-year-old have, it's called isolated speech delay - meaning the ability to understand very well, but unable to express himself in words. Some of the reasons for this could be exposure to multitude of languages (hence not knowing which word in which language to use, especially if all the languages are being introduced from the same person/source), not finding the need to speak (us as adults offering whatever they want immediately, meaning there's no need for him to speak out what he wants) etc.
If you're concerned about his language or development, the best advice I could give would be to check it out with your child's paediatrician :) I didn't think anything was wrong with my son either, but we just went ahead to get a referral to a child development unit and done a development assessment - which is where we found out he had isolated speech delay, he had the motor skills and understanding ability of children his age (at 20 months old) BUT his language expression skill (talking) is that of a 10 month old. Since then, we've been doing speech therapy sessions and it's been improving :)
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