Ha ! My Polish wife speaks a very good French with almost no accent. Learned French in school over 6 years and was very good. When she came in France, couldn't understand a word anyone was saying. All the 'chelou', 'zarb', 'relou', 'dégueulasse', 'meuf', 'flics', ... It felt like learning a whole new dirty lexicon. But since everyone uses it more or less...