Traditiones Custodes states that
... possess a knowledge of the Latin language sufficient for a thorough comprehension of the rubrics and liturgical texts, and be animated by a lively pastoral charity and by a sense of ecclesial communion. This priest should have at heart not only the correct celebration of the liturgy, but also the pastoral and spiritual care of the faithful;
Maybe this is illicit in the first place and therefore a non-issue motu-proprio-wise, but I've gone to several masses in Spanish for the Latino communities who live around me where the priest has a very weak grasp of Spanish and delivers his homily in English. Also, I've gone to many masses in English with missionary priests, who we're grateful for coming into our diocese to help with the priest shortage, but it's not clear that they posses a knowledge of the "English" language sufficient for a thorough comprehension of the rubrics and liturgical texts.
One reason I'm asking is because in recent weeks I've heard about the deep spiritual significance of Latin and that some people believe proper diction is essential to proper celebration of the Eucharist. Now, I would imagine this is not something that the Holy Father has on his mind when he's considering reasons for limiting celebration of the Traditional Latin Mass. However, he used knowledge of Latin as a bullet point, and it would seem to me that knowledge of the language the Mass is prayed in is important no matter which books a priest is using. So is knowledge of Latin more imperative than knowledge of other languages?