Your ad can show to a user when your targeted language matches:

Your ad can show to a user when your targeted language matches:

  • The language of websites a user visits most often
  • A user’s browser setting
  • A user’s Google interface language setting
  • A user’s operating system language

Answer :  A user’s Google interface language setting

Source: https://support.google.com/adwords/answer/1722078?hl=en