shooting NTSC in Europe
The two principal reasons for shooting a region-specific frame rate are (1) reasonable synchronization with environmental electrical and lighting systems (thereby minimizing flicker) and (2) reasonable compatibility with playback devices and their assumed frame rates.

That said, there are reasons people want to shoot "off speed".  It is quite popular these days to shoot at altered frame rates and/or shutter angles for aesthetic purposes.  Such creativity requires a little knowledge about how those changes are going to be affected by environmental electrical and lighting systems, and how those choices are going to play when finishing the footage to be played at the given frame rate of a particular market.

Shooting at 120 fps in an environment lit with lights fed by 50Hz mains is very likely to result in unpleasant light power phase problems.  Shooting at 120 fps in sunlight and slowing the frame rate down to 25Hz will provide 20% more slow-down than shooting at 100fps, which might be desirable for your subject (and not a problem for other reasons stated above).  If you are shooting exterior night scenes lit by sodium, mercury, xenon, and/or neon lights, you are back in trouble again.