Those 'electronic transformers' put out a square-ish wave of a few kHz frequency, often amplitude-modulated by (twice) the line frequency. Halogen bulbs don't care. Most LED bulbs do, since they either have ludicrously slow, cheap, rectifiers or filter networks that block it. You will need either a real iron-and-copper transformer, a 12VDC converter, or an 'electronic LED transformer' that works at 50/60Hz. If the latter, try it first with the bulbs you are going to use. Nothing else will ensure compatibility. The whole situation is a complete mess.
And don't waste your time with LED bulbs using a capacitative dropper - the first fast transient spike on the mains will go straight through and blow them up. Been there, done that!