what was the font in which the French albums were initially published in and why a different font for english?
I believe that the original French-language versions were hand-lettered by Hergé himself, in the way that most solo strip-cartoonists worked in those pre-computer lettering days and in which many still do. I'm not sure whether, as Hergé's studio team grew, the lettering duties were ever carried out by other people in his team in the way that some of the colouring and background/technical drawing stuff was, or if he always did the lettering himself. Someone else on these forums may well know more on that point. But in any case, the lettering style was Hergé's own, and it's very much that style - rather spindlier and less dominant, perhaps, than Hyslop's - that the current digital font is aiming to emulate.
Any translations made back then would, of course, have to be hand-lettered from scratch in the new language (unless they'd opted for mechanically typesetting the speech balloons which would have been hard to make look right even with a relatively informal typeface, and no doubt technically trickier to make fit than getting a calligrapher like Hyslop to do it by hand) so to describe either Hergé's or Hyslop's lettering as a useable font in the modern digital sense obviously wouldn't really be accurate.
I'm not sure if other non-French language publishers employed people with a hand-lettering style more similar to Hergé's own. But presumably the UK publishers Methuen actively liked the look of Hyslop's lettering, perhaps feeling that its rather literary non-comicky style helped their efforts to distinguish Tintin from the kind of comics that there was so much prejudice against in the British market. (Not that Hergé's own hand-lettering style looks particularly comicky either, but maybe a little more so than Hysop's.)
Somebody these days could, of course quite easily create a useable digital font from samples of Hyslop's lettering (modern comic-strip and picture book creators do that all the time with their own hand-lettering) and re-set the Tintin books in that. But I think it's worth remembering that while to us British Tintin book readers of a certain age the Hyslop lettering looks like the proper and correct lettering for these books, to readers of the Tintin book in their original French the Hyslop lettering style would look quite wrong, and that the current digital font being used may well look a much better match for Herge's own lettering style and thus a more sympathetic match for his drawings too.
This may also be true by now for younger British readers of the books who haven't grown up with the Hyslop lettering, and if that's the case, I'm not sure that even a UK-only petition to the UK publishers Egmont for a return to Hyslop-style lettering in the UK editions would have much logic or be met with much support.
That's not to say that I don't think the current digital font could be improved. I think I'm right in recalling that Moulinsart already made some tweaks to their first attempt at the digital font some years ago, following critical feedback, so no doubt they could keep improving it further, perhaps to make it look a little more hand-drawn still.
And I think there's definitely a case for raising the artistry of the digital lettering of some the books' sound-effects
to match that of the hand-lettered originals, which (as I think fellow forum moderator Harrock n roll tackled in another thread somewhere ages ago!) has been done rather clunkily by the modern digital typesetters - and unnecessarily so given the sophistication and subtlety that's possible with today's digital lettering when used expertly.