I'm studying Spanish for work and I can't get it out of my head the way I just know Katsuki would sit with flashcards to help.
Every time you messed up he'd bop you on the head with rolled up papers. You'd begin internally screaming when you messed up something he'd taught you a billion times.
"DÓNDE! MEANS! WHERE! Get it through your skull brat." Katsuki exasperates, on round 25 of vocabulary. It's been an hour and a half, the only word you can't seem to get through your head is dónde.
He's sure he's learned more than you have at this point, mind racing with vocabulary and phrases. You'd still need a guide while he'd probably be chatting with a local, asking where the damn airport is.
After a few more failed attempts, he'd put the flashcards down and start massaging at the roots of your scalp to relieve tension. Muttering little nothing's of "you need water and rest" by the end of it.
Katsuki would make mental notes to start watching Spanish television shows, rom-com or not, and quiz you.