Feature summary (what you would like to be able to do and where):
Option for falling back language variant interface messages to the language base when not localized

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Example: Projects have a base language (for example 'en').; Should the project localize a message (e.g. MediaWiki:Message) for the benefit of their readers and users, but a user selects an interface language variant (e.g. en-gb) - those users will not see the MediaWiki:Message content - they will only see MediaWiki:Message/en-gb. This is fine, when that message is initiated, however if that message is not initiated they don't fallback to the variant base, but to the mediawiki default message - missing out on localized data.

Benefits (why should this be implemented?):
Provide consistent localization to users of a project that select a variant.