...I have no idea how much of the, like, endorsements/etc culture actually exists, and how much is basically just down to QMs making it up, honestly.
In the manga the game is described as having "a small following in Japan" in the chapter it's introduced.
Then you have Death-T arc where holograms are introduced and it's a big deal with lots of kids there to watch Kaiba's match against Yugi, but I mostly put that down to Kaiba inviting every orphan he could find to play in Kaibaland for it's big opening.
Then the Nationals occur, and by this point it's grown enough as a phenomenon to be televised. Is the game itself blowing up, or is this mainly because holograms are the hot new thing and Kaiba flat out refused to license out the tech for anything that isn't this card game? Hard to say. The timeslot is relatively decent, since the gang are all able to gather to watch, but this might indicate it's a late afternoon thing rather than a real prime time slot, and no idea if this is some niche game culture cable channel Grandpa's signed up to that usually airs chess replays or high stakes poker.
Then there's duelist kingdom, and it... is weird. $200,000 is a lot of money, but it's a small tourney, and it's not televised or anything, as far as we know. Also appears to be exclusively Japanese competitors, which would be odd considering how recently nationals were. But then you realise all that weirdness is explained away by Duelist Kingdom being pegasus' pet project specifically to face Kaiba in a duel and beat him so he can gain control of the company (which then gets reshuffled a little when Yugi defeats Seto first at Death-T) so it was never about promotion or profitability.
Likewise, Battle City is specifically a pet project for Kaiba to lure in the Rare Hunters and win the god cards. This is just more rich guys doing rich guy things without concern for profits because fuck you I'm rich.
Keith's exhibition match against Pegasus that filled a literal arena, had an enormous wager and was televised all over America indicates to me that back at home it must be a major thing with big deals and sponsorships, since no holograms means people attended or tuned in to watch two men literally play with cardboard, but I'm just not totally certain how big it was in Japan. Kaiba really helped accelerate it though, so after DK I can see endorsements starting to become a thing, though perhaps not super quickly.
And then by DSOD Kaiba's efforts appear to have turned Domino into a magic & wizards Mecca where it's a really big thing. So by then definitely big money in it, but mainly just because Pappy Kaiba give money plz.
Edit: Forgot to talk about the R spinoff, which establishes the existence of the Card Professors Guild (in the original manga it's only Keith who is referred to as the Card Professor, one of his nicknames showcasing how good he is). This bit of info just reinforces what we already knew though, that M&W is big in America and you can make a ton of money on the circuit if you win big, which is the whole reason the the professors guild exists.
Why they all speak fluent Japanese despite being American is something you gotta just handwave away. Pegasus was eccentric and a travelling businessman so he could easily have picked up the language. His adopted sons were Japanese so obviously they speak it too, and maybe they're why he learned it in the first place. Two of the card Professor Guild members were also adopted by Pegasus, or at least his proteges, so they probably learned the language from the Tenma twins, and it disseminated out.
My theory as proposed in this quest is that after Death-T and the reveal of holograms, serious competitive pros around the world probably guessed that Japan would be the next hotbed of M&W profitability and rapidly studied the language. This is why Cedar is decent at Japanese but has some phrases he can't quite pin down, and Tilla can barely string together any phrases in Japanese that aren't literally statements from the M&W Japanese rulebook, she has focused solely on learning the language to a level of "can passably play the game with some pointing and miming". It helps that since the game started in America most of our canon pro Japanese duelist's probably learned early from card translations and are familiar with the English terminology for the game, so the Americans can default to "summon" instead of "Shokan" for instance.
