Do you not think God is smart enough to understand your prayer ?
Of course God understands. He also recognizes when we deliberately choose our words carefully, reflect on their true meanings, and use them with sincerity.
Like in the above example of the Kyrie Eleison, using the Greek is not for God’s benefit, as He needs nothing from us. It is for our benefit, so that we may better come to understand His nature and to praise it worthily.
Depth of meaning can be lost in translation and over the course of time, so when someone says “Lord have mercy” in the Mass now, they may not fully understand the origin of the phrase or the full spectrum of what is trying to be conveyed. God does not benefit any more or less, as there is no change in Him. Rather, it is us who are changed when we seek to worship Him to the fullest extent possible.
Using ancient languages like Latin or Greek to pray helps to unite us with our ancient faith. It is an honorable endeavor to seek to learn and understand the languages of our faith and the love they can convey.
Seeking to understanding Latin, Greek, Aramaic, Hebrew, etc… also helps us in our reading of scripture and the writings of the Church Fathers. Can’t we just use English translations? Sure… but why would you want to limit yourself to only these and skip over or miss some of the linguistic treasures hidden within these ancient words?
Without understanding some of these words and languages, we become unable to defend some of our beliefs, like Peter as the “Rock” in Matthew 16 (and the difference between the Aramaic כיפא (kepha) and Greek πέτρος (petros) and πέτρα (petra)"), or Mary being completely transformed (κεχαριτωμένη) by grace. What about Jesus’ “brothers” and Mary’s perpetual virginity? Without understanding the broad scope of the word ἀδελφοὶ, we would have a contradiction.
Words matter, and seeking to understand and use them well is a worthy endeavor.