I just checked, you are right, it is not multi-voltage just 230v 50Hz. The only concern that I would have using a transformer is that they tend to output 220v 60Hz and I am not sure how that would affect the output. I would be concerned as the charger has an internal switching power supply and if it receives 60Hz 220v instead of 50Hz then can it regulate the output down to the correct voltage of 36v. There are technologies that will do this but they are really expensive, I am thinking about the waveform regenerators that will give you a perfect 50Hz sine wave out that some people use in hifi.
Just a side note, I have bought bosch powered bike both in the UK and the US. Strangely my US based bikes came with a 110/220v charger and my UK ones are mainly 220V. I just wonder if they actually produce a 220/110v that would be ideal for you.
The minimum I would do in your position would be to get a 110v to 230v transformer then measure the output of the charger. If it is over 36v then I would don't use it.