Protogermanic

[HorusLupercal]

Reconstruction of long dead languages has been going on for a while in the linguistic field. I never complete much study in that (I was an anthropology major for a while in college) i'm not sure how they do it, but even an almost mythical language like indo-european (theoretical origininator of that family of languages) are being "reconstructed"

Edit: celtic must be even harder as very few celtic languages exist today and some groups of that language family are completely extinct (gaulish for example)