Who knows, and why do you care?

That's something that the highly paid engineers at Google should figure out, not something that random Internet people on HN should have to figure out when replying to your comment.

My point is that we've had 5 people suggest different mutually incompatible, and all subtly broken, solutions to this "simple" problem. Perhaps the solution Gmail settled on wasn't done so out of naïveté.

Five people just throwing ideas off the top of their head in their free time. That only makes Google not having any solution with years and billions look all the worse.

prefix the localized bin for users with an existing bin

What? We're taking about tags, so if I create a custom "bin" label and move to england, what do I type to see my trash?

Reserve a system label prefix e.g. '_' and forbid users to create labels with that prefix. Then the labels '_Trash' and '_Bin' are exact aliases, and users can't accidentally collide with them.

Ah so now I have to search in _Trash instead of in Trash which makes label discovery and use worse and confusing for all users.

No. The point is that the search language has to be localized. When you type in:bin in en_GB or in:trash in en_US, the system has to work out that you mean _Trash. Doesn't seem that hard.

But what if you have a custom label bin or trash?

In en_US 'Trash' is an alias for '_Trash' but 'bin' is not.

But yes, it's getting complicated. I can see why they just stopped with changing the GUI labels and left the workings in canonical US mode.