[Mediawiki-i18n] Fallback sequences

[Seb35]

Hello,

Now Lua has access to the internal fallback sequences of MediaWiki [1], so  
this can be used to gracefully degrade the translation of the templates  
included in a page, by the means of {{Translatable template}} (also called  
{{TNT}}) or auto-translated templates which use {{Fallback}}, mainly on  
Commons. I worked on this yesterday [2].

But I send this message mainly to show the comparison between the fallback  
sequences of Commons vs MediaWiki [3].

I guess some fallback sequences of Commons could be added to MediaWiki:
* af -> nl
* br -> fr (not sure)
* ckb -> fa
* en-gb -> en
* fy -> nl

There is one conflicting fallback sequence:
* co -> fr (Commons) or it (MediaWiki)

And some differences:
* pdt -> nds, de (Commons) or de (MediaWiki)
* nds -> nds-nl, de (Commons) or de (MediaWiki)
* nds-nl -> nds, nl (Commons) or nl (MediaWiki)

And more importantly, the purposes of the fallback sequences are a bit  
different inside a same language family/variant family: MediaWiki  
specializes from general to variants (often by script), but Commons does  
the opposite: degrades from variants to general. I find a feature could be  
added in the Translate extension to give access to these "reverse fallback  
sequences" by comparison to MediaWiki’s ones. This could be implemented  
either by creating a small database (perhaps add the opposite parameter of  
$fallback in the MessagesXX.php) either by computing these. This could be  
used for the translation of the templates, in Special:MyLanguage [bug  
48292], and to redirect the translators to the general language (the  
messages "Translate in zh please." in $wgTranslateBlacklist of  
CommonSettings.php in Wikimedia config).

[1] https://gerrit.wikimedia.org/r/101910
[2]  
https://meta.wikimedia.org/wiki/Module_talk:Template_translation#Fallback
[3]  
https://commons.wikimedia.org/wiki/Template_talk:Fallback#Comparison_of_the_fallback_sequences_between_.7B.7BFallback.7D.7D_and_MediaWiki
[bug 48292] https://bugzilla.wikimedia.org/48292

~ Seb35