This is because "Trash" is just a label. The "Send to trash" button simply is a shortcut to add the trash label.
If you called that the "bin" label in England, then functionality could break when the user switches languages... Now they have a bin and a trash, with different things in!
Or you could write the code to relabel all the users emails when they changed language...
But what if they had already manually created a label called "bin" then the auto-re-labeler will cause loss of data...
There is no perfect solution here... Any solution has unexpected behaviour for some things a user might want to do. Considering that, I think what is there is one of the least bad solutions...
If you called that the "bin" label in England, then functionality could break when the user switches languages... Now they have a bin and a trash, with different things in!
Or you could write the code to relabel all the users emails when they changed language... But what if they had already manually created a label called "bin" then the auto-re-labeler will cause loss of data...
There is no perfect solution here... Any solution has unexpected behaviour for some things a user might want to do. Considering that, I think what is there is one of the least bad solutions...