Apple *should* sue Xiaomi in any country Xiaomi is selling, except China..where they would waste a ton of money on a lawsuit that is pretty much guaranteed to fail, for reasons other than the company's HQ being in Beijing city proper's CBD with close gov't support. Xiaomi's entire concept, at the beginning, was to duplicate an iPhone at a far lower price point..wait, not just an iPhone, but *all* of the products Apple puts out there, look at their line-up. They have copied presentations, commercials, even Jobs himself, so UI similarities are to be expected.
I am curious whether Apple even cares. Xiaomi competes in the 'shanzhai' level of pricing, very low priced devices aren't really in the same park as Apple's premium priced devices, even though the Xiaomi's actually have reasonably valued hardware for what you're paying....so, while Apple *should* be suing them from a principal standpoint, maybe Xiaomi just isn't competing for the same customers in a significant enough way yet or something?