"Frankfurt Airport" will do on www.bahn.com.
The signs at the station are also bilingual.
If you're lazy you can query with the DB timetable
the official station names are
From: Mannheim Hbf
To: Frankfurt(M) Flughafen Fernbf
The direct ICE trains from Mannheim go all to the Fernbf (Fernbahnhof - long distance train station). There is also a Regionalbf (Regionalbf - regional train station) at the airport.
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