When do you refer to the page "MediaWiki:Gadgets-definition" misunderstandings may arise.
1) Should I use the namespace MediaWiki even if I have declared another one in my LocalSettings.php (eg: $wgMetaNamespace = "Iubar";) ?
2) Is the page Gadgedts-definition valid for any no US/UK locale installation of mediawiki ?
(eg: in Italian the translation of Gadgets is Accessori)
1. Changing $wgMetaNamespace does not affect MediaWiki: namespace, they're separate things
2. Yes. You just use it exactly as you see it (in English) irrespective of the wiki language.
$wgMetaNamespace controls the name of the namespace that’s called Project on mediawiki.org, not the one that’s called MediaWiki. The name of the namespace MediaWiki can be localized on non-English wikis (through software localization, not in LocalSettings.php), but MediaWiki always works as an alias, so you can just type MediaWiki:Gadget-definition in the search on every wiki in the search field, and it’ll find the right page.