It means that no need to be a smart ass to learn his
own ancestor language.
This is how I see it.
How Kozuki learn : Oden father told Oden this

is symbol of a god (using native Wano language to explain) Oden gets it directly, easily. Move on to other symbol/letter....
How Oharas, an outsider learns : What is this

? Looking for a references and found that it refers to "GOD" (in a ancient language) , What "GOD" means in our language ? And Again looking for other references.. etc... etc.. repeat on every letter they need to learned.
See? Learning someone else language is way more difficult then learn our own ancestor language.