Add fallthrough to "parent" language

[Jikoo]

Reduce load on server owners who want to modify language files - easier way to do bulk edits.
_{"I'm making it easier for server owners to change languages," he says after pushing towards using more resource-pack-based translations. "This is good," he says.}

TODO

• Move languages to a subfolder in the file system
• Use en for default instead of en_us
• Is reloaded with config reload (not that OpenInv currently offers a way to do this, but hey)

Closes #85