Please consider maintaining (y)our own fork of Prettify.
I just tried to add new keywords for Swift¹ but
- they wanted me to sign a CLA with all kinds of personal information and
- the project does not seem to be maintained actively; there are many outstanding issues (140 as of now) and pull requests (26) and the last activity on language support was 11 months ago.
In other words, Google Prettify is not actively maintained and their policy discourages community contributions (compared to other projects).
I'm confident that an SE-fork of Prettify, maintained by the community, would be much more active and current. For keeping both forks in sync, SE could act as a single contributor towards Google Prettify.