> Like when you say an English sentence to a Mexican who doesn't understand it, and you respond by saying the same thing again but louder and slower. It looks like you think they're dense.

My wife is learning Spanish right now because we are moving to Costa Rica. Her most used phrase is “de espacio, por favor” which means “slowly, please”. She’s able to understand much more of the language when folks slow down and annunciate clearly.

When I lived in Chile as a teen, I also asked people to do the same thing frequently until I became proficient. I still ask sometimes in other Spanish-speaking countries because their accent or dialect differs from what my ears are most used to.

Speaking slowly and clearly to anyone learning a language is incredibly useful and is not insulting in any way.