Because the Roman Empire had troops there. Mostly foreign trops from everywhere in the empire, from Spain, Syria, Lebanon, Germania, etcetera. Of course the administrative language was Latin, although like in all languages the Latin of, say, the fourth century AC was not anymore the sculptured language of Cicero. The common soldiers communicated in a mixture of Latin and native languages. Married or cohabitated with women in the regions where there legions or cohortes were stationed. After the shrinking of the empire these influences remained.
"est" is third person, singular and "tu" is second person, singular. I keep getting the plural and singular mixed.
Singluar, Active: Quis sum ego? Quis es tu? Quis est ea?
Plural, Active: Quis sumus nos? Quis estis vos? Quis sunt eae? <-- someone help me out here (ei (ii), eae, ea)?
No. Quis tu es, or Quis es tu.
Estis is with vos.
And Es is with tu.
Quis es = who are you (singular)
Quis estis = who are you (plural)
/Quis vos estis/Quis estis vos.
Like in "Quis estis vos qui tentatis..."
Almost the same than the French es->es.
And estis ->êtes. (ê meaning a former "es" spelling)