I'll try to point you to specific titles. There's a whole movement dubbed "La Nouvelle Manga" (after Nouvelle Vague). It's European, by European artists, but it either adopted fully the Japanese style, or merges the franco-belgian style with the Japanese. There seems to be a lot of Fantasy themes, and the characters look suspiciously like the love-child of Asterix and Candy.

As for popularity, perhaps Belgium is different (it would be like Québec, then) but I've heard it was nearly as strong as in France.

Manga is not a mainstream phenomenon in the US. It's increasing its sales very slowly, though it's starting to pick up pace a little and several publishers of comics have started publishing mangas too. There's something dubbed American manga, but contrary to the phenomenon in France/Belgium, it's for the most part not really American. It's done by Japanese artists for American publishers.

In France it has passed into mainstream culture toward 1995. The Japanese themselves have dubbed the phenomenon "The French exception" as it yet the only country aside from Japan itself where this has happened (and depending on the source, francophone Belgium is or isn't specifically included). Québec is a bit behind.

Depending on who you talk to, they explain the phenomenon a bit differently. Most credit the very strong bédé culture, with its strong storytelling tradition, which made manga quite less exotic when they appeared than they were to Americans. Others mention some cultural affinities between bédé and manga, for instance the tradition of anthropomorphism. The franco-belgian tradition is similar to Japan's, and both are quite unlike the American tradition (from Hannah-Barbara to Disney) that belongs more in the comedy genre only. Europeans were also already familiar with creatures populating an universe of their own, with bédé like Les Schtroumpfs, which were mainstream back then.

Another frequent explanation was the omnipresence in the 70s-80s of anime (not called that back then) on TV in France, Belgium and Québec. They nearly eclipsed both the European studios's productions and the American ones. The European animation studios quickly reacted to the new competition by making co-productions with the Japanese studios, commissioning them to produce stories for Europe. Perhaps you're too young to have known any of them, but if not you might be suprised about some series you probably think are European but were actually designed and produced by Japanese studios. Calimero, Heidi, Le Petit Castor, Belle et Sébastien, Les Mystérieuses Cités d'Or, adaptions of Les Trois Mousquetaires, Les Misérables, Sherlock Holmes (as Sherlockhound) and so on. All japanese productions, with European involvement. The first manga to appear in France were those on which the anime series were based (then Akira, the first smash hit). They were really succesful, and publishing housesstarted to introduce other series that had not been broadcast as anime.

Anime went in decline in the 90s, apparently because of a cultural misunderstanding. Japanese were quite taken quite by surprise that all of sudden manga were in high demand somewhere abroad. So far it had been mostly a fringe phenomenon,like a few titles becoming cult ones (like Akira) in niche markets here and there, mostly among professionals of comics, for instance. In the franco-belgian market there was a kind of small niche for adult bédé, but it was ingrained that animation is for kids. That's really different in Japan, where there's a great deal of anime targeted at 16 y.o. and up. As the manga sold, the TV stations asked for those anime, but broadcast them in slots for kids. There was much controversy apparently, those anime being deemed to violent or mature, and the Japanese productions got discredited and blamed as being insuitable for European kids (Ségolène Royal notably published an essay denouncing how the Japanese anime culture was getting more and more violent and risqué...). Of course, the Japanese company didn't understand that in France an animated series can only be for kids, and the Europeans didn't know Japan produced series not quite intended for kids. In any case, the European animation studios pretty much stopped co productions and returned to European productions. The audience had gotten a taste, though, and that's when the manga boom truly kicked off.

Despite being described as "mainsteam" in Francophonie, it's still somewhat more like a big sub-culture I think. I never much noticed it myself until I started talking with people into it, and reading some. Bookstores have a manga section, but no more (I never even knew about that before getting interested in the genre. At first I also went along the preconception that forcibly the choice would be better in English, and the books cheaper too. Neither was true, and as manga sell much more in French, the tables are kind of turned for once and it's in French the printing quality is much better, and the books a few dollars cheaper than the American ones). To find most of them, you need to go to specialized bookstores. There it's really huge... a few thousands titles in stock, sometimes as many as 50 titles coming out each month. Another thing that makes "the French exception" is that several genres/titles have become very popular in France that have not been bought anywhere outside Japan yet (not even Korea and Hong-Kong). Literary adaptations for adults, adult dramas, avant-garde, even a manga exploring oenology (its massive success in France is actually weird and a glaring exemple of how mainstream manga has become that the French audience take it in stride as if it was a French work, when they are extremely defensive about any work about wine coming from abroad normally) culinary-themed mangas and more. Some authors are much acclaimed in France/Belgium, getting bought by the bédé publishers from the manga ones (for more expensive and mainstream editions, similar to the bédé editions), that have yet attracted no other interest outside Japan. A newer phenomenon still unique at this point is that some Japanese mangaka whose talent get recognized in Europe (mostly guys with no publisher in Japan who publish their work on the web) and who have problems emerging in Japan are getting signed for original series by the French publishers that then sell their work to Japanese publishers.

Money-wise it's the same, of course. As of now, the most lucrative market for manga outside Japan is the France-Belgique-Québec combo, followed by Korea, then HK, Brazil and Spain, Germany and only then the US.

As for influences, it's even in Astérix now (there's a manga joke in one of the recent albums).

About bédé I'll be back, I erased the first part of my answer by mistake and it's getting late here.