LED's are current dependent devices, to use them something has to limit the current or they would run away and burn them selves out, there are three basic ways to turn the current dependent device into a voltage dependent device, the circuits that convert from current to voltage dependent are called drivers.
1) A resistor, these produce heat but with a 12 volt device the LED is typically going to have 3 volt across them so 3 LED's and a resistor works quite well.
2) A capacitor, as long as frequency is steady with AC a set amount of current will flow through a capacitor, so this is a cheap method of control, normally also a few resistors to ensure there is no voltage remaining in bulb when you remove it, and to bleed some power without lighting LED's so they will not store a small amount of power from natural capacitive or inductive linking and flash every so often, and also it allows dimmer switches and the like to be able to work without a neutral.
3) A pulse width modulated controller, these are used to make the lamp more efficient, and also allow a larger voltage range, I have a LED bulb in outside lamp rated 85 - 300 volt, the other two return around 75 lumen per watt, but this method 100 lumen per watt or more. However they will not work with dimming switches, or any other device before them which modifies the supply, which includes another pulse width modulated supply to drop voltage unless fully smoothed.
In the main one of the three above is build into the bulb, although you can get external drivers. But in the main once packaged into a bulb the bulb is voltage dependent. However if the bulb is marked 50 Hz then that is what should be supplied, as we have no idea what is inside the bulb, again if marked DC 10 - 30 volt then we need to supply it with ripple free DC in the main from a battery.
The problem with 12 volt is although most have a simple resistor, we as the used don't know for sure if that is the case, so if marked 50 Hz then only way we can be sure of correct supply is a wire wound transformer, when we use a switch mode or pulse width modulated supply, likely the frequency is in kHz range, and if it does have a capacitor to limit current which is I admit unlikely, we can't use DC or kHz supplies, it has to be 50 Hz.
The rules at moment say if not dim-able it must be marked as such, so if the unit says not dim-able it is likely it has a pulse width modulated driver inside so is a better quality product.
There is also a problem that with a kHz supply it could become a radio transmitter, so not a good idea even with a unit rated 0 - 60 VA to use it to supply a LED. With units rated 20 - 60 VA unlikely you can find a LED to draw enough current.
This problem working out what can power what, means easy way out is simply to use 230 volt.