Plebs tibi, Domine, Virginique Matri dicata, beati Simonis solemnitate laetetur: et sicut per eum tantae protectionis signum obtinuit, ita praedestinationis aeternae munera consequatur.The "sign of such great protection" is a reference to the brown scapular. When Our Lady appeared to St Simon, giving him this distinctive Carmelite clothing, she said that whoever died devoutly wearing it would not suffer the fire. Promises such as these are never intended to be a "get out of jail free" card but an assistance to those who devoutly use such sacramentals. Hence the prayer asking humbly for the blessings of eternal predestination, not a presumptuous claim.
O Lord, may the community dedicated to You and to the Virgin Mary rejoice at the solemnity of blessed Simon: and just as it obtained through Him a sign of such great protection, so may it gain possession of the gifts of eternal predestination.
I translated "plebs" as "community" following the late Latin use noted in Lewis and Short. It conveys the sense that there is a particular community that is dedicated to Our Lady of Mount Carmel, including both the Carmelites themselves and those who wear the brown scapular.
It is a great blessing to be so near Aylesford. It takes me about minutes to drive there from Blackfen so I sometimes call over when I have an opportunity, especially to join the community for Saturday compline and the Carmelite anthem "Flos Carmeli" (more information at this post.)