The Death Eaters all seem to be British, possibly even Karkaroff.
There don’t seem to be any known foreign Death Eaters. It’s possible that foreign Death Eaters exist, but the known Death Eaters either are clearly shown to be British or their nationalities aren’t something that can be reasoned from what’s known about them. The majority of the Death Eaters, and sometimes their relatives as well, were known to attend Hogwarts, implying that whatever their original heritage was, they themselves were likely British. Even the one who seems most likely to be foreign, Igor Karkaroff, shows signs of not being nearly as foreign as his position as headmaster of Durmstrang might imply. When Karkaroff was convicted, he was tried for his crimes in Britain, and sentenced to Azkaban, the British wizarding prison.
“He was caught, he was in Azkaban with me, but he got released. I’d bet everything that’s why Dumbledore wanted an Auror at Hogwarts this year – to keep an eye on him. Moody caught Karkaroff. Put him into Azkaban in the first place.”
- Harry Potter and the Goblet of Fire, Chapter 19 (The Hungarian Horntail)*
His case was also tried in Britain, further implying he’s originally ‘of Britain’.
“Very well, Karkaroff,’ Crouch said coldly, ‘you have been of assistance. I shall review your case. You will return to Azkaban in the meantime …”
- Harry Potter and the Goblet of Fire, Chapter 30 (The Pensieve)
When he visits Hogwarts for the Triwizard Tournament, he calls it ‘dear old Hogwarts’ and says it’s good to be there, which implies a certain previous familiarity with Hogwarts.
“Dear old Hogwarts,’ he said, looking up at the castle and smiling; his teeth were rather yellow, and Harry noticed that his smile did not extend to his eyes, which remained cold and shrewd. ‘How good it is to be here, how good …”
- Harry Potter and the Goblet of Fire, Chapter 15 (Beauxbatons and Durmstrang)
Additionally, though Krum is clearly depicted as having an accent, Karkaroff isn’t - he speaks perfect English and is never shown speaking another language.
“But ve have grounds larger even than these – though in vinter, ve have very little daylight, so ve are not enjoying them. But in summer ve are flying every day, over the lakes and the mountains –’
‘Now, now, Viktor!’ said Karkaroff, with a laugh that didn’t reach his cold eyes. ‘Don’t go giving away anything else, now, or your charming friend will know exactly where to find us!”
- Harry Potter and the Goblet of Fire, Chapter 23 (The Yule Ball)
From the evidence we have, it seems more likely that Karkaroff was originally from Britain and then fled abroad after being released than that he’s actually at example of a foreign Death Eater. As no other Death Eaters seem particularly likely to be foreign, it’s very likely they were Britain-based.