Localisation is too hard for Gmail
This is because "Trash" is just a label.  The "Send to trash" button simply is a shortcut to add the trash label.<p>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!<p>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...<p>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...