It will never stop being weird to me that you anglos just translate his first name to Frederick / Fred instead of going with Friedrich / Fritz, it's as if i was calling Biden and Stalin "Jupp" because that's the traditional short form for Joseph in the Rhineland.
Wait i've actually called Stalin Jupp in the past nvm
We have evidence of this as early as the 1838-9 letters to his sister Marie (Engels was in the habit of using random bits of English even in his earliest letters).
And it becomes more prominent ofc once he moves to England and lives there. Marx himself used 'Fred' to refer to Engels often, including in this Dec. 6 1868 letter
Afaik this is the transcription of the original letter rather than a translation (it can be found in MEWBand32). I'm not a German speaker, but my guess would be that any weirdness is a combination of:
it being 19th century German;
it might have traits of 19th century Rhineland dialect;
it uses random English;
it's a speedily written letter so may have errors
Marx tends to use random English, French, Latin, Greek, Italian, etc words and grammar in his personal writings along with his own contractions in German, and just words he's made up
Honestly, here in Poland we use polonized names for the theorists and they were also polonized historically.
Karol Marks, Fryderyk Engels, Włodzimierz Lenin, Józef Stalin and so on.
Including even some soviet politicians and more people presumably, but most just have their names phoneticized for polish speakers. If a guy is of a Polish origin, it's guaranteed their name will be written fully in Polish. Like Feliks Dzierżyński and Róża Luksemburg for example.