Having taken a single linguistics course in college, I'm in no way qualified to answer this but will take a stab, anyway, with respect to PIE -

PIE is a completely reconstructed language and so is largely hypothetical, as I understand it. There are no examples of PIE usage that remain (again, so far as I know). So, while scholars have educated guesses about the language's mechanics, and also have a sense as to how certain words would have looked/sounded at some point in the distant past, they would certainly not be "fluent" in PIE if they were able to travel back in time and speak with someone steeped in that language. Reconstruction of PIE is based on a comparative study of words across existing languages that generally mean the same things, so if an original PIE word did not have any descendants in later languages, then I imagine it would be hard for modern linguists to ever know of that word. Not only that, but as we all know, a word can have many meanings in the same language depending on context, the fashion of the times, etc. - linguists would not be able to capture that level of detail about reconstructed words, or be able to trace how PIE changed over time or by geography with any great degree of precision.