Does it guarantee to do this for everyone or just Europeans? Genuinely curious here since they might have separate policies in place for different countries- I imagine that China prefers to keep all its citizens' emails indefinitely.

Google has been doing this for years, before even making public guarantees. There wasn't a separate policy for China when I left years ago and Gmail is blocked there anyway.

How do I know where my mailbox is located??

It's actually complicated to know where your mailbox is. Even if you knew its current locations, Gmail rebalances the N replicas across X data centers all the time, based on a number of factors.

unless there is a secret law that prevents them to actually do it...