There's a cool graph...
...from...
https://www.mediawiki.org/wiki/Manual:Language#Fallback_languages
Some languages in MediaWiki have what is known as a "fallback sequence". This is where MediaWiki will fall back on a different language if it cannot find what it needs. An example of this is the language code frc (Cajun French), which falls back on the language code fr (French). The reason for this is that sometimes languages don't have all messages defined in them.
The fallback for a language can be found in its associated languages/messages/MessagesXX.php file.
EN has been our direct default for any strings w/o a translation in a given lang, but seems pretty useful to attempt getting string from more closely related lang to user lang (or article lang, etc) if one is available...