I am referring to the song Despacito by Luis Fonsi, featuring Daddy Yankee. Fonsi himself has declared that the English translation is "Slowly", but when describing why he chose the word "despacito", he describes sentiments that really are not implied by the English word "slowly". I think "nice and slow" is better on several levels. Adding the suffix -ito to the adverb in this case makes it sound warmer, more casual, and more positive. I think the English equivalent is something like "nice and slow". As an added bonus, this is an extra syllable and it is easier to stretch it out, as is done in the Spanish vocals. If you want the syllable count to match exactly, you can do "nice and slowly", but as a native English speaker that doesn't sound as natural to me.
The song title should probably remain "Slowly", because Fonsi is the artist and he wants that to be the English title. In addition, when "despacito" actually modifies a verb, I would leave it as "slowly" in English - for instance, "I want to breathe your neck, slowly". However, when "despacito" occurs by itself, not in a phrase, I think it should be "nice and slow".
Fonsi has not actually translated the entire song into English (as far as I know), so I think he might have used "nice and slow" in context if he did so. Fonsi has actually said that he doesn't think the song works in English, he thinks it is too silly.
I looked at all 8 English versions on this site, and they all use "slowly" as the English word. I suggest "nice and slow".
Different question: In the Spanish transcription, it has the line "aquí tengo la pieza, oye!". In the album version, the final word (which may or may not be "oye") is on a different vocal track, it is overlaid on the track of DY singing "aquí tengo la pieza". Perhaps it should be in parentheses because it is a separate vocal track? Also, I am really not convinced the word is "oye". On the album version, it sounds very much like the English word "slowly" (at 2:29 on the album version). When Luis Fonsi performed the song live on Conan, he either said the Spanish word "oye" or the Spanglish/English interjection "oh yeah". In other live performances, the word has been omitted entirely. I also might suggest omitting the word altogether, it is a stylistic flourish that doesn't fit the verse structure and is obviously optional in performance.