I believe that they are US sizes. I've ordered a few super cheap bras to test out the sizing, and so far it's about 90-95% right on. (The panties? Not so much. WOMEN HAVE HIPS, PEOPLE! Jeez.) If you're concerned about that, though, order larger than you think at first, and either fit into it right the first time, or learn from it and then grow into the bra!
Most, if not all, of the sellers actually include measurements if you scroll down below the item in question - cm for band, etc. - and many of them also include conversion charts there too, which include sizes from around the world. Most of them are also super responsive with questions: you can ask them for specific measurements and they will likely get back to you in a quick enough manner that it insinuates some poor worker is up at all hours of the night...
I haven't had to do this (YET) but if you go by the conversion chart they have on their store page, and the bra really doesn't fit, I believe you can open a dispute and either get a full refund (if you want to send the cursed thing back) or a partial refund (and not have to pay shipping).
I hope you find something that works!