Update titles in metadata and syntax

[gkellogg]

as suggest by @iherman in #469 (comment):

The title property in the syntax is defined as "any number of human-readable titles for the column, each of which has an associated language represented as an object whose properties MUST be language codes as defined by [[!BCP47]] and whose values are arrays of strings related to that language." Is this correct, grammatically? But, more importantly... I personally would have preferred to keep it as it was, i.e., something like "any number of human readable titles for the column, each of which with an associated language as defined in [[!BCP47]]". However, if we do that change, the metadata document has to change, to (for natural language properties) so I did not do any change.