[Jellby]

Originally Posted by carmenchu

Please, are there language codes for Old Indic, Avestan, Old Church Slavonic, Old Norse, Old English, Old High German, Phyrgian, Hittite, Luwian...?

You could refer to:
https://www.loc.gov/marc/languages/language_name.html

Old Indic: USE Vedic: Assigned collective code [san]
Avestan: [ave]
Slavonic, Old Churh: USE Church Slavic: [chu]
Old Norse: [non]
Old English: USE English, Old (ca. 450-1100): [ang]
Old High German: USE German, Old High (ca. 750-1050): [goh]
Phrygian: Assigned collective code [ine]
Hittite: [hit]
Luwian: Assigned collective code [ine]

I had some "fun" translating all language names in calibre...