The properties plugin's locale option needs a language tag such as "de-DE" or "es-ES". An invalid tag, such as "en_US" or an empty string, is ignored: dates render in English and the browser rewrites them in the reader's locale.
This invalid tag falls back to the reader's locale:
import properties from "comark-properties";
const plugins = [properties({ locale: "en_US" })];Use a valid tag to fix the locale:
import properties from "comark-properties";
const plugins = [properties({ locale: "de-DE" })];Or leave locale unset to use the reader's locale.