well he's been on dates with random men dressed as a woman so they can hear him (he has a phonecall with one of them)
http://familyguy.wikia.com/wiki/Stew...tanding_Stewie
Quote:
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Brian and Jillian are the only adults that seem to fully understand what he's saying; however there have been occasions when other characters have evidently understood him:
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There's also the time Stewie brainwashes Chris and Chris has to do as he commands, so assumedly he understands him. There's a list of examples on that link