modifier.soulstealer.desc:

• Soul Shards: The Old Ways
  modifier.soulstealer.desc=Additional souls per kill for your Soul Shards

• TinkeringWithBloodMagic
  modifier.soulstealer.desc=§oRidiculously wicked!§r\nSteals the souls of enemies and crystallizes it into demon will.

I think the forge doc can help your https://mcforge.readthedocs.io/en/latest/concepts/internationalization/#usage-with-blocks-and-items

Unlike registry names, translation keys are not namespaced. It is therefore highly recommended to prefix your modid somewhere to the translation key (e.g. examplemod.example_item) to avoid naming conflicts. Otherwise, in the event of a conflict, the localization of one object will override the other.