Never once mentioned to be a different nationality or have different ancestry
It's never mentioned that he is Belgian, either, nor is it even certain that he's speaking French, given that he could be speaking Flemish... It's often uncertain what language the characters are intended to be speaking: the Scottish characters all speak in fluent French to Tintin in the original Black Island
, when presumably it's Tintin speaking English to them; so Haddock and Tintin could be speaking Flemish to each other, but it's being translated like the English was in Black Island
(or they could
be speaking English - it's not possible to be definitive).
Anyway, that's the point of this discussion - it's looking for clues, for nuance - Tintinology is meant to be fun, after all.
This question of nationality isn't something that arises out of the books being translated into English either - it's based on the anecdote of Hergé's about the "sad British fish", the haddock. That's our hint.
Archibald is used in other countries
Again that holds equally true the other way - Archibald is a name more common in Britain than Belgium; the fact that it could be found in Belgium doesn't negate the possibility that Hergé intended him to be British.
We have George and Paul Remi
Not certain of your point here - for a start, Hergé's name was the distinctly Francophone Georges
, rather than the English George. If your suggestion is that you can't tell what nationality someone is from their name, then that just takes us back round again to the start - if we can't assume that someone called Paul is Belgian, it's just the same for someone called Archibald Haddock.