Format used in JSON input and output of Wikibase REST API uses kebab-case spelling for fields that have multiple-word names, e.g. ...property.data-type.
This format is quite inconvenient to work with for a number of programming languages, incl. JavaScript.
Let's use a format that is equally readable but better to work with for programmers. Fields with names consisting of multiple words should use underscore as a word delimiting character ("snake case")

Fields to adjust (list might be incomplete):

• property.data-type -> property.data_type (in statement, qualifier etc data)
• data-type -> data_type (top-level field in data of a property)
  
  "language codes" defined in Medawiki/Wikibase that use dash in the identifier. (e.g. zh-hans, pt-br) to be kept as is. Those are used as JSON field labels