* mejor: means "better" or "it's better"
* hablame: is a conjugation of "hablar" (to speak), meaning "speak to me"
* en español: means "in Spanish"
* sii: is a misspelling of "sí," which means "yes"
Putting it all together, the phrase roughly translates to: "It's better if you speak to me in Spanish, yes?"
Here's a more grammatically correct way to say it:
* "Mejor háblame en español, por favor." (It's better if you speak to me in Spanish, please.)
The original phrase might be used in a casual setting, perhaps between friends, but it's not grammatically perfect.