Marx did read English proficiently and did write some academics in English, but in his excerpts he would use German as main language. Marx would however often work together with people of whose the first language was English or French etc. when translation something.

Marx bewegte sich ziemlich sicher nebst der deutschen Muttersprache im Englischen, Französischen und Spanischen. Es wird ihm von Zeitgenossen für diese Sprachen grammatische Korrektheit nachgesagt. Es wird ihm freilich auch seine mangelhafte Aussprache vor allem des Spanischen angekreidet

Marx was very secure in English, French and Spanish in addition to his German mother tongue. He is said by contemporaries to be grammatically correct for these languages. Of course, he is also blamed for his poor pronunciation, especially of Spanish