For Latin, the go to resource to start learning is the book Lingua Latina [1] which has an easy conversational style. You can understand a good percentage of the book without any a priori (see what i did there? :) ) knowledge of any kind. Concurrently with that, you might listen to the Latin language videos by Scorpio Martianus [2] who also does some videos in Greek.

[1] https://books.google.com/books/about/Familia_Romana.html?id=...

[2] https://www.youtube.com/channel/UCRllohBcHec7YUgW6HfltLA

I think that might be quite a challenge.

The Rosetta Stone was a huge breakthrough find, as it was an important clue for hieroglyphs, as it had the same decree written 3 different times.

Linguistic anthropologist s usually learn from many hours of field recorded conversations for nearly extinct languages.

The Bible is the arguably most translated book in history. If the Bible has a translation in one of your dead languages, you could start there.

Even Machine Learning needs language pairs (same text in two languages) to get started.