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:

1. it being 19th century German;

2. it might have traits of 19th century Rhineland dialect;

3. it uses random English;

4. 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

It's German, it sounds weird even on the best of days