We expect all language fallback chains for Terms (i.e. Labels, Descriptions, and Aliases) to end with ('en').
These chains are constructed in \Wikibase\Lib\LanguageFallbackChainFactory\Wikibase\Lib\LanguageFallbackChainFactory.
This is consistent with current expected behaviour and also core https://github.com/wikimedia/mediawiki/blob/master/includes/language/LanguageFallback.php#L95
- adjust TermLanguageFallbackChain itself to always return at least en
- add a test case to TermLanguageFallbackChainTest that tests with a language that is not a valid language code (such as an emoji or ⧼Lang⧽)
This has already caused the following incidents: