All Windows 10 images already contain .NET 3.5 LangPack pre-integrated and staged, you only need the main cab to enable the feature that en-us cab file is just leftover from UUP .NET 3.5 LP function is exactly like .NET 4.x LP, it's just for UI messages/controls, they don't affect .NET apps
Thanks for getting back to me. I'd still like more information. What is UUP and what was it used for? Why include it if it's not needed?
UUP is the reason that en-us cab existed originally and still, .NET 3.5 LP is part of the main Windows LP