Old Baltic Prussian language is still alive :
The Æsti, mentioned by Tacitus in his Germania, may have been a people who spoke Old Prussian. Tacitus describes them as being just like the Suebi (a group of Germanic peoples) but with a more Britannic-like (Celtic) language.
Old Prussian was closely related to the other extinct Western Baltic languages, Curonian, Galindian and Sudovian. It is more distantly related to the surviving Eastern Baltic languages, Lithuanian and Latvian. Compare the Prussian word seme (zemē),[1] the Latvian zeme, the Lithuanian žemė.
Old Prussian contained a few borrowings specifically from Gothic (e.g., Old Prussian ylo "awl," as with Lithuanian ýla, Latvian īlens) and even Scandinavian languages.
If anyone interested in Learning this language: There are still people who can speak it in Lithuania,Germany in todays (Kaliningrad),Poland...etc there is prussian gramma and vocabular releast...and you can join this web site there are people who can help you learning this langauge.
Baltic Prussian forum