What's the difference between installing packages and software from the main server and from the local country server?
The software packages on each mirror are identical.
You can test this for yourself by checking the hash of any package from different mirrors. The hash will be identical.
Local mirrors are a convenience for you provided by volunteers (not Canonical). A local mirror may (or may not) offer faster downloads. You can use any mirror in the world that you can reach.
Since mirrors are volunteers efforts, new mirrors join and old mirrors drop out. Occasionally a mirror may fall a day or two behind...but be patient: It will catch up when the local problem is resolved.