I think I have an idea that should address different concerns raised by different individuals.
In the short term: we get the current schema validator in EventLogging compliant with Draft 3 and move it into core. This would require the least work, and other things rely on Draft 3 so by sticking with that we cause the fewest breaks. When using this validator, you would need to specify Draft 3 for future compatibility. (For those depending on EventLogging for validation work, the schema validate method would just invoke this core validator with the Draft 3 argument.)
In the medium term: we work with an upstream Draft 4-compliant validator to implement i18n support and then incorporate that alongside the other one into core. Draft 4 would be a separate mode that you would need to pass as an argument. Draft 3 would be deprecated.
In the long term: we work with whoever we need to work with to get an actual JSON Schema standard adopted and work with upstream on implementing that standard. "Standard" would be implemented as the default mode in the core validator, and Draft 4 would be deprecated.