The ability to adjust a user-interface to the user's local language or dialect
Because I live in the UK, I speak en_GB (English, Great Britain) rather than en_US (English,
Simplified United States).
Mostly, all dialects of English are mutually intelligible. Sure, the Brits love the letter U and the Americans stick a Z in every possible word. But we get along reasonably well. Except in Gmail.
Here's my en_GB localised Gmail interface. Note how there is a folder called "Bin".
Everyone using Gmail in en_GB will know that deleted emails go into the "Bin".
Gmail has a handy search feature to allow you to find emails in a specific folder. For example "Bob in:spam" finds all email containing the word "Bob" in your spam folder. "Proposal in:sent" gets everything you've sent with the word proposal.
But it is impossible to search the "Bin" folder.
Why? Because you have to search the Trash folder. Because that's the names used by Americans.
The same is true even if you've chosen a non-English language.
Sadly, Google don't respond to user complaints or feedback. The best you can do is hope a ranty blog post gets high enough traction on social media. Then, maybe, something will change.
If you're building a service - remember that localisation is about much more than the GUI. All aspects of the interface need to be considered.