The following quotation is attributed to Pope Pius XII:
The day the Church abandons her universal tongue [Latin] is the day before she returns to the catacombs.
Says who? A few notable uses of the quotation online are:
Cream City Catholic in 2013, in a fact check article (that does not source any of its quotations)1 Tridentine Catholic Website, in their header image2 Praying Latin, who source the quotation as “days before his death”3 and Traditio in 2005, the earliest instance I can find of the quotation online, who source it as “Mediator Dei”4 Mediator Dei and Pius on Latin Pius’ encyclical Mediator Dei is the source that is most commonly given for this quotation, but one will search in vain for the quotation in that document.