From a reader:
Is there any reason why one cannot use Latin for only one of the ordinary prayers of the mass in the OF? In other words, may the Agnus Dei be in Latin while the rest are in the vernacular? Estne aut omnia aut nihil?
In the Ordinary Form you can always use Latin for any part of Holy Mass; all of Mass or only parts of the Mass.
There is great flexibility for music in this regard as well.
There is no reason why you could not have an Agnus Dei in Latin.
As a matter of fact it is difficult to justify not having parts of the Mass in Latin.
Latin is the language of worship in the Latin Church.
The Second Vatican Council’s Sacrosanctum Concilium 54 requires that pastors of souls teach their flocks to sing and respond in Latin and their mother tongue.