Error message i18n support

[differentmatt]

This would probably follow the GitHub locale file model used in CodeCombat. Clients will need to pass in the target locale to Aether.

Things to consider:

1. Are native runtime error messages localized?
2. Are any transpiled messages localized?
3. Our error hints are hard-coded
4. Some of our error hints are based on matching hard-coded error messages
5. What is the pipeline for i18n contributions?