We welcome any localization for axe-core. For details on how to contribute, see the Contributing section of the main README. For details on the message syntax, see Check Message Template.
To create a new translation for axe, start by running grunt translate --lang=<langcode>. This will create a json file with the default English text in it for you to translate. Alternatively, you could copy _template.json.
To update an existing translation file, re-run grunt translate --lang=<langcode>. This will add new messages used in English and remove messages which were not used in English.
_template.json is a generated file which is created every time axe is built. It's compiled using each rules' description and help properties as well as each checks' metadata.messages property. To update the _template.json file you'll need to update the corresponding rule or check metadata file and rebuild.