Usage notes

• Set the key path before or after selecting language files. Join the parts in the key path with a period (.).
• For example, to get the stage name "Scorch Gorge", the key path should be `CommonMsg/VS/VSStageName.Yunohana`.
• The file name should be in the form JPja_full.json or JPja_full_unicode.json.
• The file format should be the one as in Lean's splat3 repo.