You’re in good shape with languages. For the UN, definitely increase your fluency in French.
It might be fairly difficult to pursue the UN with a journalism/advertising undergrad background, so a master’s degree is a good idea. Master’s programs in, say, political science or international relations or psychology (for childcare) might prove difficult to get accepted to with your academic background.
Perhaps the best path for you to transition into UN/Humanitarian/NGO type work would be to focus on your cultural strengths, with Asian/East Asian studies. Here is a list of programs (which include master’s degrees): https://www.asian-studies.org/programs/bystate.htm
You could also try to find a master’s program in communications, with an international focus, like this one: http://www.american.edu/sis/ic/index.cfm I don’t know how common these types of communication programs are, however, that offer an international/global direction, which would be more appropriate for your career goals.
Also check out this useful career info: http://www.coyotecommunications.com/stuff/workabroad.shtml