How do i politely ask my husbands side of the family not to "baby talk" to our 5 month old son?

Baby talk: Perhaps posting articles on the issue on the fridge or laying them on top of coffee table/end table reading material is a way to get your point across. Just be sure to choose well-supported articles with documentation.

Let me think . . .: Having practiced pediatrics for over 20 years, i can say that every culture I have encountered has a special way of talking baby-talk to babies, toddler-talk to toddlers, and so on. It is normal for humans to speak to preschoolers, grade-schoolers, middle-schoolers, teenagers, and college students all in manners different from one another. It would sound very strange if people did not do that!

No harm done: You may not be able to get others to change their behavior. If you can, great, if you can't, don't sweat it. Baby talk is not going to harm your baby's language development. It still has a close resemblance to adult language. The important thing is that people are talking to your baby (rather than ignoring your baby)! s/he will learn a ton from the social cues.