Check out http://www.itqm.ch
. I don't know anything about them, but they seem to be an international partner of ASQ (the American Society for Quality), which I CAN vouch for, having been a member for 5 years.
ITQM (Institute for Total Quality Management) offers a ton of training courses (some in English), but nothing titled 'Lean Six Sigma Green Belt'. If you really want that title/certification, check out ASQ's online courses and/or their certifications (http://asq.org/learninginstitute/index.html
). You can take the test for the Green Belt in Zurich.