Is there any evidence of non-British wizards attending Hogwarts?
I don't remember any direct statement in the books saying that some student does not come from Britain.
There is some canon evidence that wizards can attend foreign schools though. Draco says that he considered going to Durmstrang but his mother opposed because it was too far away:
… Father actually considered sending me to Durmstrang rather than Hogwarts, you know. He knows the Headmaster, you see. Well, you know his opinion of Dumbledore – the man’s such a Mudblood-lover – and Durmstrang doesn’t admit that sort of riff-raff.
*Harry Potter and the Goblet of Fire Chapter 11: Aboard the Hogwarts Express.
One more indirect evidence can be found in Harry Potter and the Deathly Hallows. Lupin says there that this year it is mandatory for all children to go to Hogwarts and that is did not use to be like this. Some parents were allowed to educate their kids at home or abroad:
‘Attendance is now compulsory for every young witch and wizard,’ he replied. ‘That was announced yesterday. It’s a change, because it was never obligatory before. Of course, nearly every witch and wizard in Britain has been educated at Hogwarts, but their parents had the right to teach them at home or send them abroad if they preferred.
Harry Potter and the Deathly Hallows, Chapter 11: The Bribe.
So it sounds plausible that also non-British students are allowed to study in Hogwarts.
They can not
Can American kids go to Hogwarts?
J.K. Rowling: No, they have their own school. You'll find out in Book 4. Hogwarts just serves Britain and Ireland.
J.k Rowling does mention off book that there are schools of magic in North and South America but unrelated to European schools. There was talk of fan fiction being allowed that took place in other countries including the states.
Given that Hogwarts is connected to the Flu Network, It's very plausible that foreign witches and wizards may attend if they chose tho. JKR mentioned that there are other schools located in different countries but its a matter of preference (and spoken language probably).