2.9.2 Update and Core files

Hello!

Yesterday i was asked to upgrade my site to a new version (2.9.2) and i did it and everything went just fine. Since my site is running in spanish i got asked again today to upgrade since the spanish file was available.

In the past i upgraded my site first in english and then in spanish and everything went fine, but in this time, everytime i update my forum with the spanish package, after it says it all went fine, the little message to upgrade my site it comes again. No matter how many times i upgrade the site, no matter wich btrowser im using, it simply ask again to upgrade.

Wich i did is upgrade my site with the english version and hide the spanish version wich have a problem or so. But this question comes to my mind:

If i upgrade my site in english first and then in spanish (like i allways do) ?Some trash code are added to the core files? or ?When WP updates, it only replaces all the files for the new ones? ?What is the correct process to upgrade a site runing in other languaje but english?

And one more question:

When i install a plugin, the code of this plugin rewrittes something in the core files? or just create its owns folders and archives and run besides the core?

Thanx!