Originally, the Mass in Eastern Europe, where it started, was celebrated in Greek.
With time, as Christianity spread further West into Europe it was celebrated in Latin.
And as time moved on, it is now celebrated in English, or whichever language is spoken in the Country where Mass is celebrated.
Except for three words which have survived time and are still from the original Greek Mass.
Meaning "Lord have mercy" and "Christ have mercy".
It is important to remember that by "Lord", or "Sir" we do not mean that we look up to Our Lord as some Master or Ruler. In this context, the word "Kyrie" means "Lord" in the sense of a child looking up to one's loving parent and asking for help, love, guidance and protection.
So when we sing at Mass "Kyrie Eleison" we look up to God our loving Father and ask Him to be always by our side and have mercy on us. And we call upon Christ His Son, in the same prayer.