i18next.com/pages/translate/interpolation/locales/en/translation.json

i18next website

{
  "key": "{{what}} is {{how}}",
  "keyEscaped": "no danger {{myVar}}",
  "keyUnescaped": "dangerous {{- myVar}}",
  "keyDeep": "i am {{author.what}}",
  "keyDifferent": "__myVar__ is important"
}