the new math: old battle of the lovees was: PDP1 3576

Of course it depends on the individual. But based on the number of such people I've met, I think I can start to get around to "seem to ... on average". Plenty of counterexamples too, sure.

So far I haven't gotten a single false positive looking for the English-speaking Welshman or the Finnish-speaking Lapp, for example. Plenty of other people have, though, so I guess I've just been lucky but ...

It may be more of a case where after the third language the accents get mixed up beyond recognition, but still, say, ethnic-Swedish Finns who are also fluent in English, are rather harder for me to spot in any of their three languages. Likewise the Lapps who speak a lot of Norwegian too, but then I don't speak Sami nor much Norwegian either...

Well, you're non-average then ;-) but nothing new either.

The really odd ones are those who seem to catch certain local accents in no time and then hang on to them even after getting home...

It depends. It was sort of funny the first few times my mother was mistaken for a local in Lapland and she had to try to explain that she only spoke Finnish ... but some of my wife's relatives keep managing to avoid the tourist surcharges and such in Russia.

-- #Not speaking for my employer. No warranty. YMMV.