Claude and Jonathan are from an original world setting, but they're one of the rare character I actually kind of thought real world nationalities too to some degree
In Claude's case, the Lovelace family is from a Britain-inspired country, and he is a Volkov from his mother's side, who in turn are kind of Finnish-Russian inspired? So IRL he'd probably have some sort of British-Russian roots with some Finnish to the mix in terms of heritage.
Jonathan in turn would definitely be Swedish, like there is just something about him that makes me think of our neighboring country and I'm very content with that headcanon.