May did well in school generally, and this is a thing that's taught in school, rather than something trainers would teach themselves based on how much they care, so I'm not entirely convinced May would necessarily be particularly bad at Pokémon speech. It's a subject, she learned it like everyone else, and she's good at learning. I was bored to tears by Danish at school but I still got very good grades, and that didn't even have the kind of overlap with any of my interests that Pokémon speech has with Pokémon training. (I mean, surely it's useful as a trainer to be able to ask your Pokémon questions.)
However, in general the fic just intentionally relegates the Pokémon speech worldbuilding to the background. I've said before that I basically just want you to pretend the Pokémon are speaking English; there is never any kind of instance of a human not understanding what Pokémon are saying because of the language barrier (not even Mark's parents, who would surely have significantly less reason to know Pokémon speech than May does), because what I wanted here was just everyone being able to talk to each other. To facilitate this, there's imaginary fantasy education that makes everyone fluent in Pokémon speech, done.
In the IALCOTN I gave the Pokémon speech worldbuilding more actual presence, and in that kind of setup it'd be appropriate to play with exactly how good each given human character is at Pokémon speech. It's fairly likely I'd do something like that in the next revision. But I think it'd probably distract from May's story if she were struggling too much with the language. It's not about the language, and though it'd be nice to show she's not as good at it as Mark, I wouldn't want to have a language barrier significantly affecting her thoughtlessness towards Larvitar, or getting seriously in the way of her talking to Stantler.
(And oof, maybe I should add a spoiler tag.)
COMMENTARY DONE