A plugin-authored error — error_code doubles as an i18n lookup key (the frontend's apps/frontend/src/i18n/locales/*.json use literal English sentences as keys, not symbolic codes; a missing translation falls back to rendering error_code itself verbatim, i18next's own default behavior). Write error_code as a natural, stable English phrase that does NOT embed any dynamic value (a file path, a URL, an HTTP status code, ...) — those go in data instead, so the same error_code can be looked up and translated regardless of which specific file/URL/status triggered it. E.g. for "Could not open foo.json!", use error_code: "Could not open file" + data: {filepath: "foo.json"}, not the interpolated string itself as the code.