Hi again. As indicated on https://wiki.openstreetmap.org/wiki/Key:destination#Value

it is much preferable to use the actual destination "as somebody (including navigation) would speak to you"

"St." can be read as "street", "sankt", "saint", "es tee" ... you name it. Destination in this case "Sankt Gallen" whi is the ... actual destination. What is written on the sign or on map of the official city is "St. Gallen". Similarly another example would be "Bruck a/d L." or "Bruck a/d Leitha" or "Br, a.d. Leitha" means "Bruck an der Leitha" as destination