Thanks for the answers! I'd seen the site linked, so I was broadly aware of what had been published; I was more looking for the kind of context you've offered.
It sounds like Spirou is somewhat less well-known in Finland than in Norway, where I'm from. There the series first appeared in 1973. The publishers were first "Forlaget for Alle" and Interpresse, later Semic and Egmont. Under the title "Sprint" it was published in albums at a rate of about 2 per year all the way up to the mid-nineties, at which point it had caught up to the present and almost everything had been released. There were also frequent reprints of earlier albums, and Semic/Egmont's comics book club "Seriesamlerklubben" collected the whole series (or practically all of it).
Due to the lack of new material (as I'm sure you know, after 1995 there was only one new album for almost ten years, 'Kone joka näki unia,' as Tome & Janry focused on Le Petit Spirou), the album series then practically ceased, but they started to run reprints in 'The Phantom' magazine (Fantomet) instead, so it didn't totally disappear.
After Morvan & Munuera relaunched the comic in 2004, the album series resumed, and they even put out the few albums that had been missing so far (Nic & Cauvin's three entries, and the one with 4 early Franquin stories). But we only got two of the "one-shots" (Yoann & Vehlmann's and Tarrin & Yann's), and Egmont seems to have given up on them. The Norwegian album market has shrunk a lot, and nowadays you can't find them in most supermarkets or newsagents', the way you used to. The main series still comes out (in hardcover as of the last album), but it's one of the few remaining album series, and I don't know how long it will last.
Instead, Egmont released a version of the French intégrale book series, collected volumes of Franquin's Spirou + a little bit of Rob-Vel and Jijé (in Swedish, Norwegian and Danish). Did you get that in Finland? Unfortunately, they decided not to continue with Fournier or the later authors.
So that's what I thought was a bit strange: Spirou is historically one of the most popular and well-known European comics in Norway, but its current status is marginal. It doesn't seem like it was ever quite as big in Finland, yet now it seems to be doing better in your country. Maybe your comics market is just in better shape generally?
Anyway, I don't think co-printing can explain why you've got albums such as 'Nuoren sankarin päiväkirja,' 'Tummanvihreä pikkolopoika' or 'Paniikki Atlantilla,' since to my knowledge Egmont hasn't published them anywhere else.