Thread:Project:Support desk/Add a dialect/reply (3)

There is no need to alter so many files. Beyond $wgExtraLanguageNames, you only need language fallbacks (in MessagesEs_mx.php). For ULS, the files is langdb in https://github.com/wikimedia/jquery.uls, or you cna hack directly jquery.uls/src/jquery.uls.data.js

This supersedes all your other points.

As for your point BUT(2), see assistant languages.