The foundational languages of Italian and Hindi are Latin and Sanskrit, respectively. So much was known about them, probably because they were liturgical languages. Then why isn't much known about the foundational languages of the ancient Germanic tribes? Celtic tribes? Slavic tribes?
I am afraid that the concept of a “foundational language” is not terribly useful. Modern French is very different, for instance, from the form of proto-French that was used in Strasbourg in 843 and recorded by the Carolingian historian Nithard, to the extent that a modern speaker of French would certainly not understand his 9th-century counterpart; as such, they are different languages for all practical purposes. But proto-French was also different from Latin. In turn, Latin had antecedents (proto-Italic, proto-Italo-Celtic, proto-IE…). A more scientific concept is the idea of a common ancestor. Romance languages are defined by their common “ancestor,” Latin. Similarly, the Indo-European, or the Afro-Asiatic macro-families are defined by the existence of a once common language (though it was of course spoken in a much smaller area that the modern one).
Our main problem is that we do not have early testimonies (inscriptions, texts…) about each of these proto-languages, especially the more ancient ones. You contrast Latin with Proto-Germanic in your post: but the last time “Proto-Germanic” was spoken may have been in 250 BC, whereas the fragmentation of Latin apparently did not occur before 500 AD. Another important factor is geography: to restrict myself to Europe, let's say that literacy tended to spread towards the Mediterranean coasts at first, and then towards the North. The farther off a proto-language is from these centres, the less likely it is to have been written down in early times.
In rare cases, there is none at all, but we have enough early data to construct a convincing picture of the ancient language (e.g. Proto-Indo-European, thanks to very early texts such as the Rig Veda, the Avesta, or even our impressive corpus of Greek, Latin and Persian literature). However, in most others, not only our earliest inscriptions post-date the end of the common language, but they are also very sparse in the early periods. It makes the reconstitution of this ancient tongue much more complex, though valiant efforts have been made by linguists to create a view of what these early languages might have looked like (efforts on which /r/linguistics might be more helpful than AskHistorians).