Splice

Client-side mod that merges split lang folders at reload. Flat lang/{locale}.json / .lang plus lang/{locale}/** fragments are served as one synthetic file from a virtual pack at TOP

Authoring

1.20 / KubeJS - from Modpack-Modern kubejs/assets/tfg/lang/:

kubejs/assets/tfg/lang/
  en_us/              # fragments (merged in path order)
    items.json
    blocks.json
    quests/...
  de_de.json          # flat file still works

{
  "item.tfg.antipoison_pill": "Antipoison Pill",
  "item.tfg.haste_pill": "Haste Pill"
}

1.12 / GroovyScript - same tree under groovy/assets/{ns}/lang/, extension .lang:

groovy/assets/tfg/lang/
  en_us/
    items.lang
  en_us.lang

Keys starting with __ are skipped. Later packs / later fragment paths win.