Improve LanguageManager flexibility

[Jikoo]

• [x] Add a system similar to .guess files where missing keys are listed with default language values
  • guess section containing missing entries
  • Reduce verbosity during run by just warning about the presence of the guess section.
• [ ] Add support for more generalized language
  • Slightly complicated by the fact that MC does not follow standard language tag spec. It is consistent with itself, but it allows resource packs to add whatever they want without any enforcement.
  • en should cover en_.*, etc.,
  • Default language en_us should also be mapped to en for convenience.
• [ ] Migrate language files to their own directory
  • This is done internally in 0b46f9fcc90013680c9d6e40278c02d46b89d72b, may be nice to also do in the file system