I have a bell on both my MUPable bikes and totally recommend them. Bells are easily understood no matter what your native language and are (generally) not considered offensive but rather more in the "gentle reminder" category.

The only downside, really, is if you get to where you expect everyone to hear the bell and don't notice the i-pod users who for sure won't hear it! A friend of mine (Alice Strong) recommends using two bells (maybe the two different tones get more attention?) rather than just one.

Rick / OCRR