"e.g. When applying to a job as an leveraged finance analyst covering European PE firms, you could mention that you only speak English and German fluently and are not satisfied with your proficiency in French (only at conversational level). Then show that you have been taking classes in the evening and have spent time in France during your holidays to become more proficient. "
^ imp, I like the example you cited as a response to this "weakness" question. Shows your cognizance of a weakness -- one that's important to you but fortunately not absolutely paramount to most of the aforementioned businesses (since they do speak English), so nobody will dock you for missing a critical skill. But importantly, it demonstrates self-initiative and a broader vision of your goals, i.e. to speak foreign languages well enough in order to diversify your potential addressable client base. Great suggestion.