To answer the OP…
The only document I’m aware of that “regulates” the use of the vernacular in the Mass is Sacrosanctum Concilium itself. And, if memory serves me correctly, it leaves the extent of the use of the vernacular up to the discretion of the local synods of bishops.
That being said, I believe Pope St. Paul VI, in Iubilate Deo, set out a minimum of Latin that was to be maintained (the Gloria, the Creed, the Sanctus, and the Agnus Dei, along with the Kyrie [which, of course, is Greek]).
Is it okay for a priest to switch from one language to another? Absolutely, so long as he is using approved translations. I’ve been to tri-lingual Masses offered in Latin, English, and German. I’ve been to Byzantine Liturgies offered in English, Arabic, and Greek. At my own Maronite parish we have Liturgy in English, Arabic, Syriac, and Greek! It’s pretty amazing.