Hello!
My personal feel, as a non-American, is that the term "international planning" is a bit of a misnomer. The idea of urban planning, as someone mentioned, differs from country to country.
If you're looking at the Middle East, Russia or China, an urban planner equals mostly to master planning, urban design, subdivision designs and the like. You're likely to have a strong design background, coming from Architecture, Landscape Architecture, or a strong design-based Urban Planning and Design, with quantitative skills to do simple calculations and projections. This will be further supported by other allied industries like civil engineering. This is because many of these countries are growing and the governments are trying to house the population in new cities built up from scratch, while at the same time undertaking a lot of high-tech test-bedding work. You'll find a lot of "eco-cities" springing up, with use of green building technologies and pedestrian friendly features.
If you're hoping to do more development work, i.e. organising development projects, community consultations and the like, you would require a slightly different set of skills including, but not limited to, impact assessments, development theory, community consultation skills and the like. Here, the skills you need are "softer" as you'll be doing more project management work.
I do think that it would make sense to ask yourself what type of job you do want to work overseas, as in a more technical role or more project based. If you want a more technical career, then you should build up on the technical skillsets involved in that area. Be prepared for change, and be prepared that you may not necessarily enjoy in the area you'll thought you'll enjoy working in, and new areas may open up to you.
One suggestion I can give is to also check out the NGOs/IGOs websites to see what sort of skillsets they are looking out for. Places like UN Habitat, the World Bank and so forth will describe on their websites what the person is expected to do and what qualifications they want.
Hope this helps a bit.
