According to Wikipedia [0] the headquarters of IERS (International Earth Rotation and Reference Systems Service) is in Frankfurt, Germany (contact information on their website [1] has an address in Frankfurt too). The web server hosting the linked file also contains German text in its error page:

> Dieser URL wird nicht beantwortet. Bitte wenden Sie sich mit Ihrer Anfrage an das Bundesamt fuer Kartographie und Geodaesie.

[0] https://en.wikipedia.org/wiki/International_Earth_Rotation_a...

[1] https://www.iers.org

Ok but that makes even less sense

A mix of English and French is just called... English