It's the opposite, no? It's because it's a part of Lunargoth culture that only actual Lunargoths can pronounce them? Just because a Reptile Yokai was descended from Lunar Dragons doesn't mean their family or bloodline actually kept up with Lunar culture. Like how just because your grandpa is Hispanic doesn't necessarily mean you speak a lick of Spanish.
This would work if the lunar dragons were like people they aren't they are beastly even their true dragon form versions are a lot of time too beastly that's why I was saying it's only about lunargoth basically the dragon would be be the special beast of the hispanic in this scenario and to add on more we don't even know if the lunar dragon ancestor was a true dragon.
So the real reason is the lunar dragons even the better ones are too beastly.
So even if the mc was a lunar dragon directly he wouldn't be Able to pronounce it, but there is even moe to it ,it's like their runes,the handwraps name can only be pronounced by lunargoths the handwraps are special like they will have more content and lunargoth bros will get more out of it because they will actually be able to access it's perks.
Respectfully, I don't think that's how race and culture work, bro. That makes zero sense
It does tho it is how things work your assumption is already wrong because you thought the lunar dragons were advanced they have only recently started to learn things and are still primitive even the best among them and we don't even know if our ancestor was a true dragon version of lunar dragon.
So this is more about you assuming they were something which they are not and I guess it Isn't your fault because In game there isn't much about them but yeah they are far more beastly and are only now learning more speech is still not one of the main things for them even now they copied the shard domes tho and are making some of their own.
Also yokai kids have almost no connection with their dragon parents unlike other races lizard yokai are too much of an outcast the kid of the ancestor would have been saddled with the human parent.So there were no secrets to teach or anything and nor would the dragon ancestor be Able to talk properly.
The human or other humanoid race ancestor basically f*cked a Dragon who couldn't even speak properly lol or at all and only has the ability to turn humanoid form for s*x.
Freaky human ancestor.
TLDR: Dragon too dumb or primitive to speak let alone learn the language properly they are not pets but they were the beastly companions of some lunargoths.