Some translators appear to think that local kids would not be interested in foreign monarchs and therefore need to identify with kings and queens that they would have heard of at school. The logic is that having references to foreign monarchs might affect sales.
With respect, I think you're missing the specific point of this bit of translation. Whilst the general tendency of the English translators to Anglicise Moulinsart's Belgian setting and Haddock's French naval ancestory was indeed done out of concern for making the books more accessible to UK readers when the books were first translated (and given the much-discussed difficulties of launching an unknown comic book series in Britain back then, it arguably made sense at the time even if it seems unnecessarily parochial now), in this particular
case, surely the main reason for going with British monarchs is to make this specific joke work for UK readers.
Whilst I wouldn't have been fazed by or uninterested in a reference to French monarchs as a British child reading this Tintin book, there's no way that I (nor any average British child) would have realised that Henry XV of France wasn't an actual monarch, whereas many British kids would
know that Henry X of England didn't exist and are thus able to enjoy the joke.
So in being less faithful to Hergé's historical reference, the English translators are surely being more
faithful to Hergé's gag of having Castafiore not only guessing the wrong monarch in her attempt to seem knowledgable, but naming a non-existent monarch. I can't help feeling that this would have been Hergé's preferred priority, and an example of why he had such a high regard for (and friendship with) his English translators.
So I doubt it's a case of the Spanish translators putting more credit in their readers as you suggest. Unless all Spanish children have a full knowledge of French Kings and Queens (which frankly seems unlikely) they probably miss the joke altogether. It's possibly just a case of the Spanish translators feeling less inclined or less free to translate the spirit of the work as much as the literal content. Or maybe they missed the gag themselves.
Has the head not been picked off this scab often enough…?
You're right. Sorry for rising to the challenge! I just wanted to point out that this particular translation decision doesn't really belong in the general "Should-the-UK-translators have-Anglicised-Tintin?" argument, which as you say has probably been picked over often enough.