To say “alien” in German, meaning a being from outer space, means wrapping one’s tongue around the word Außerirdischer (m.), which literally means “extraterrestrial”.
In German “Earth” is Erde (f.), with the adjective irdisch used to describe anything belonging to this planet or being on this planet (weltlich, on the other hand, means “worldly”, “secular” or “mundane”).
While unterirdisch and überirdisch mean “below ground” and “above ground”, respectively, außerirdisch is clearly for things “aside from” or “outside of” Earth.
Nowadays the English noun “alien” (“Alien” (n. or m.)) can also be used to describe a creature from another planet, especially in movies and/or if it’s characteristically more animal than human.
Alien as Foreigner
In the other sense of “alien”, as in “foreigner”, the German word is either Ausländer (m.) or Fremder (m.).
More rare is Fremdling (m.), which means “stranger”, although this word is usually only used in fantasy novels or fairy tales.
The German word for “alien element” or “foreign body” is Fremdkörper (m.), and “illegal alien” is illegaler Einwanderer (m.).
The adjective for “alien” (in any sense other than “extraterrestrial”) is simply “fremd”.
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Examples
The aliens have landed! Die Außerirdischen sind gelandet.
It’s an alien lifeform. Es ist ein außerirdisches Lebewesen.
Are there many illegal aliens in this country? Gibt es viele illegale Einwanderer*innen in diesem Land?