Should I acknowledge institutions where I did internships on unrelated works?
Be honest in your acknowledgments. Just don't add someone or some institute name just because you want to show your link with them. If they did help in any shape or form, well thank the institute and the exact people involve in your discussions. Something like the following: 


  I would like to thank Mr/Mrs X, from the institute Y, for their insightful/encouraging discussion on J,K, and Z.