lugna ner sig bedeutet auf Englisch

calm down▼(To become less excited)
verb
[UK: kɑːm daʊn] [US: ˈkɑːm ˈdaʊn]

cool down▼(to become less agitated)
verb
[UK: kuːl daʊn] [US: ˈkuːl ˈdaʊn]

simmer down▼(to become less angry)
verb
[UK: ˈsɪ.mə(r) daʊn] [US: ˈsɪ.mər ˈdaʊn]