American comic books were not sold commercially in the UK until the late 1950s, however some were brought over by American service men stationed here during WWII and there was a circulation of them during the 1940s and 1950s, especially in busy port cities like Belfast and Liverpool, as they were used for ballast for American cargo ships. This is also the way much American music reached the UK before commercial radio began here and it's part of the reason British bands had so much American influence in the late 50s and early 60s.
For a Canadian context, the answer is mixed. Superhero comics were imported into Canada from their inception in the 1930s: after all, Superman was co-created by Joe Shuster, a good Jewish Montreal Toronto boy. There was also the beginnings of a homegrown industry whose bread-and-butter was reproductions (read: rip-offs) of American books, but the big titles from the States were imported directly. They definitely whetted an appetite for comics (superhero comics, but also crime and horror comics and pulps, and kids' funnies, animal stories and so on).
In late 1940, the Canadian government banned the import of certain paper products from America into Canada, which included comic books and pulp magazines. The legislation was intended to preserve Canada's stores of American currency, but there was a definite classist element: glossy magazines that appealed more to the upper- and middle-classes were excluded from the ban, which thus ended up targeting the kinds of texts read by working-class adults and children. Compared with the literary positioning of the middle- and upper-class magazines, pulps and comics were considered unnecessary, expendable cultural products.
But this left a vacuum in the market. By March 1941, the first domestically-produced original comic book was published by Maple Leaf Publishing, out of Vancouver. Better Comics no. 1 was in full colour, although later issues were in black and white, and introduced the first Canadian superhero, Iron Man (not that Iron Man - this one was the superstrong lone survivor of an Atlantis-like South American lost civilization).
To make a very long story short, in the absence of American imported comic books, a small group of Canadian publishers produced dozens of titles and tens of thousands of copies of comics between 1941 and 1946. While early comics were still very much in the mode of the American books, gradually (and in part because of the intensified patriotism brought about by war) the comics and their heroes became more identifiably Canadian over time. The first Canadian national superhero was Nelvana of the Northern Lights, an Inuit mythological figure who was 'translated' into a comics superhero (by making her white and giving her a mini-skirt, but who's counting?). Nelvana was followed by Johnny Canuck, known for punching Hitler in the face in the course of his adventures, and Canada Jack, who had his inception in an educational comic called Canadian Heroes, the historical content of which was seen to need some spicing up.^1
When the war ended in 1945, so did the embargo on importing comics and pulps. Some publishers folded immediately, others tried in vain to compete with the American books now flooding over the border by upping their production values, but by 1946 there were no more domestic comics being produced: the only remaining Canadian publishers survived solely on reprinting American books. Soon after, the moral panic about violence and grey morality in comic books began, and the field shifted again with the establishment of the Comics Code Authority. But that gets us firmly into the 50s.
I don't know for sure if any American books were ever translated into French for the Quebec market, but I strongly doubt it. Quebec has typically been very protectionist, culture-wise, and I can't envision a situation in which translated American (nationalist) superheroes would appeal, let alone be profitable.
^1 By far my favorite thing about Canada Jack is his sad, whimpering end. After the war ended, storylines about saboteurs and military infiltrations became a lot harder to sell. As such, in one of Canada Jack's last adventures, the villain is.... a blue jay.
Sources:
Bell, John. Guardians of the North: The National Superhero in Canadian Comic-Book Art. Ottawa: National Archives of Canada. 1992.
Bell, John. Invaders from the North: How Canada Conquered the Comic Book Universe. Toronto: Dundurn, 2006.
Smith, Michelle Denise. “Soup Cans and Love Slaves: National Politics and Cultural Authority in the Editing and Authorship of Canadian Pulp Magazines.” Book History 9 (2006).
Edit: added a few sentences at the end.
Edit 2: Correcting my faulty information about Joe Shuster, pointed out by /u/ape_unit. Thanks!
Based on what I know about European comics, the answer for the 1930s would be "yes" and then for the 1940s "no".
In the 1930s children's magazines in Belgium and the Netherlands would often feature translations of American comics.
In the 1940s, both in Belgium and in the Netherlands, local comic artists came into their own when the supply of American comics dried up as a result of the German occupation. In the Netherlands Marten Toonder rose to prominence with his Tom Poes series (though there had been Dutch comics before, such as the shamelessly socialist adventure stories of Bulletje en Boonestaak). In Belgium, both Tintin (already fairly popular before the war) and Spirou (likewise) rose to a prominence they had not had before. But whereas before the war Dutch and Belgian comics had been also rans to imports of American work, during the war they rose to positions of great importance, as there was really nothing else.
When the war ended, they maintained this position. To this day the popularity of American comics in Belgium and the Netherlands is much less than that of the local product. Not only are Superman and Batman seen as cheap trash while Tintin and Spirou are much respected, but it's also a lot easier to get a Franco-Belge comic in the Netherlands and Belgium than it is to get a superhero comic (though the latter are often cheaper).
(The Dutch equivalent, Tom Poes, meanwhile, ended up in the realm of great literature, but as to why that happened is a story way, way outside of the scope of this question).
In short, for the low countries, yes they were, until they were overtaken by local products.