Marx learned some French and English while at school. When he arrived in England, he realized that his English was too limited. He began to study hard to improve. He improved quickly and soon began writing articles in English for an American newspaper. In fact, one of these articles was written in English so well that Engels wrote him a letter praising him. Marx wrote back that Engels' praise greatly encouraged him. However, he went on to explain that he wasn't quite sure in two areas, grammar and certain idioms. These letters were written in 1853. In the years that followed, Marx continued to learn English and to use it. By the time he wrote one of his great books, The French Civil War was, he had mastered the English language so well that he was able to write books in English.
In the 1870s, when Marx was in his fifties, he felt it was important to study Russian forms, so he began to study Russian. After six months, he reached the point where he could read articles and news reports in Russian.
In one of his books, Marx gave some advice on how to learn a foreign language. He said that when people learn a foreign language they should not translate everything into their own language. If they do, it means they haven't mastered it. When using a foreign language, they should try to forget their own language completely. If they can't do that, they haven't really mastered the essence of the foreign language and can't use it comfortably.