The Denim update is already out in Germany for Lumia 625, it isnt out in Belgium though. I'm from Belgium, can I change my region and language to Germany, download and install the Denim update, change in back to Belgium? Will it work properly?
No it will not work.
Check under SETTINGS and ABOUT what is after carrier. CV??.
The ?? Is the country variant of your phone. Only if the update is released for that country (by MS) you get the update.
But if you have a carrier locked phone you get the update when the carrier releases it.