Over the years I have seen at least one from the following nationalities, of course this is assuming that what I have been told about the nationliaty is true.
English (can't remember seeing any Scots or Welsh, but I'm sure I must have done)
Irish (just one)
French
French/Arabic
Italian
Latvian
Russian
Polish
Hungarian
Czech
Slovak
Romanian
Bulgarian
Ukrainian
Brazilian
Finnish
As far as I'm aware I have never seen Swedes, Danes, Norwegians, Dutch (saw a couple of girls in Amsterdam but can't remember whether they were Dutch or not) Belgian, Swiss, German (don't seem to be that many working here), Portugese, Spanish (considering the number working here is surprising I haven't), American, Canadian, Australian (saw one as part of a two girl, but she didn't do much so it hardly counts!)
I must have seen other nationalities I have forgotten about or when I have been told the wrong nationailty.
Going back to the orginal topic I have seen as far as I am aware three Hungarians. Two were absolute crap, whilst the third was very sexy, young and attractive. She turned me on like hell, but ultimately the service was relatively average.
Based upon my limited experience then I wouldn't go along with the orginal theory.
In my experience nationality is irrelevant, it all depends on the indvidual.
I have been told in the past by other punters though that Brazilians are crap (my couple of punts with Brazilians would bear this out) and that Latvians are better than Lithuanians (can't verify this as I don't recall seeing any Lithuanians)!