Learnable:
Coptic
Old Babylonian, Middle, New, Late; Standard Babylonian
Old Assyrian -> Middle Assyrian -> New Assyrian
Eblaite (?)
Classical Arabic
Ge'ez
Classical Syriac
Biblical/Classical Hebrew**
Ancient Greek/Koine Greek**
Latin* **
Old Galician/Old Portuguese
Old Leonese
Old Castilian
Old Aragonese
Old Occitan/Provençal
Old Catalan (splits from Occitan at some point)
Old French-> Middle French
Old Welsh -> Middle Welsh
Middle Cornish
Old Irish -> Middle Irish
Old Norse (Old Icelandic)
Old English -> Middle English
Old High German -> Middle High German
Old Church Slavonic/ Old Bulgarian**
Sanskrit**
Middle Persian
Pali/Prakrit
Avestan
Classical Tibetan
Classical Japanese
Old Tamil -> Middle Tamil
Ottoman Turkish
Chagatai
Classical Nahuatl
Classical Chinese
On the fence:
Sumerian
Middle Egyptian (no vowels)
Old Akkadian (pre sargonic -> sargonic -> Ur III) (???)
Ugaritic
Umbrian
Oscan
Gothic
Yola
Old Prussian*
Old Persian
Sogdian
Hittite
Tocharian A/B
Gaulish*
Norn*
Not-learnable:
Etruscan
Lepontic
Galatian
Noric
Primitive Irish
Celtiberian, Astur, Galaic, etc *
Lusitanian
Tartessian
Iberian
Phoenician
Moabite
Elamite
Hurrian/Urartian
Hattic
Phrygian
Ammonite
North Picene
South Picene
Venetic
Messapian
Lydian
Lycian
Sidetic
Pisidian
Carian
Illyrian
Dacian
Thracian
Volscian
Meroitic
Vandalic*
Lombardic
Burgundian*
Goguryeo
Buyeo
Silla
Baekje
Gaya
*reconstruction and/or revival movements exist
**in use as a ritual language
Feel free to make any changes.
Anyway, I'm surprised Classical Japanese is on the "Learnable" list. I could never even find a lexicon.
And, while I'm at it, I may as well add the websites for the reconstruction/revival movements:
Old Prussian:
http://donelaitis.vdu.lt/prussian/lang.htm (includes modernised form)
Gaulish:
http://www.moderngaulish.com/ (includes modernised form)
Norn:
http://nornlanguage.x10.mx/index.php?nynorn (includes modernised form)
Celtiberian: no actual website that has any information.
Vandalic: It's actually my project, but we don't have a website yet.
Burgundian:
http://onska.lescigales.org/index.html (includes modernised form)