`language` of `TextQuoteSelector` are missing

[gautierchomel]

That would affect the values of prefix, exact and suffix.

Refering to 1.3.2.1. Text Quote Selector

[gautierchomel]

Also, consider that a TextQuoteSelector can be a large text, including language shifts that need to be preserved if present. For that, the value 'plain/text' must evolve to support the identification of portions in the language. See discussion #16 and #21.