Looking in the Help file, the procedure for adding a new language is explained in Chap. 9 "Language-Specific Editing" at "Adding and Removing Languages".
Basically, it assumes that the new language can be defined by choosing an existing language that is the closest in terms of syntax (thus can be assimilated by an existing parser/tagger), then modifying all the details such keywords, formatting/coloring settings, etc., until some kind of useful result may be achieved.
A very quick glance at examples of the Google "protobuf" definition language shows similarities to JSON syntax, so maybe that's a starting point...