Hi, can i buy an iPhone 5 in Hong kong and use it in Belgium?

Sure, but bear in mind, your warranty/support will only be valid in Hong Kong. If you're OK with those limitations, have at it.

You'd be better served purchasing an iPhone in Europe...EU wide warranty.

The only unlocked model sold in Hong Kong is the A1429 GSM model. That's the same model that's sold in Europe. The A1429 CDMA model is not sold in either Hong Kong or Europe.

You don't want the A1428 GSM model...its not sold in either Hong Kong or Europe.

So if I buy an iPhone in Germany, I have warranty in Belgium?

if it is an unlocked-sim version, you can use it in Belgium

Yes!

but you have to buy it from an official reseller and in case of repair give it to an official reseller.

And if I buy it online?

also online

Why are you listening to Guballu's posts? None of them answer your question in any way, shape or form.

Any iPhone, purchased in any EU country, has an EU wide warranty. Required by law.