What logic can allow a convert to faithfully with peace of mind
practice a religion in a foreign language that he does not understand?
The Quran is in Arabic because it was revealed to 7th century Arabic speakers.
Yet, the fundamental message of the Quran has always been universal: Oneness of God, doing good, and being held accountable for what we do.
Surely those who believe, and those who are Jews, and the Christians, and the Sabians; whoever believes in Allah and the Last Day and does good, they have their reward with their Lord, and there is no fear for them, nor shall they grieve. (Quran 2:62)
In all different scriptures and their languages, this is the same message.
God is the creator of languages and is not contained in one speech. In all tongues a person can contemplate, understand, and pray to God.
"The seven heavens and the earth, and all beings therein, declare His glory: there is not a thing but celebrates His praise; And yet ye understand not how they declare His glory! Indeed He is Oft-Forbear, Most Forgiving!" (Quran 17:44)
What language did Abraham pray to God in? It wasn't Arabic and he was not Arab.
What language did Moses pray to God in? Not Arabic.
What language did Jesus pray in?
What about Adam, Enoch, Isaac, Ismail and the others? They were not Arabic speakers.
Prophet Solomon spoke the language of birds (Quran 27:16), the ants (27:19), and Jinns/Genies (27:39). Did they all speak Arabic?
Same with David.
"So We made Solomon to understand it; and to each one We gave wisdom and knowledge; and We made the mountains, and the birds to celebrate Our praise with David; and We were the doers" (Quran 21:79)
Arabic is not the basis for prayer; remembrance of God is the basis:
Indeed, I am Allah. There is no deity except Me, so serve Me and establish prayer for My remembrance. (Quran 20:14)
And the purpose for this remembrance is righteousness/piety/preservation:
"O mankind! Serve your Lord, who created you and those who were before you, so that you may be righteous." (2:21)
We will only be held accountable for what was in our capacity.
Allah says, as it comes in translation:
Allah does not charge a soul except its capacity. It will have what it has gained, and it will bear what it has earned. "Our Lord, do not impose blame upon us if we have forgotten or erred. Our Lord, and lay not upon us a burden like that which You laid upon those before us. Our Lord, and burden us not with that which we have no ability to bear. And pardon us; and forgive us; and have mercy upon us. You are our protector, so give us victory over the disbelieving people." (Quran 2:286)
I should add, even early non-Sunni scholars like Abu Hanifah permitted praying in Persian and his later followers in different languages (Nasafi, Kanz al-Daqaiq).