We expect all language fallback chains for Terms (i.e. Labels, Descriptions, and Aliases) to end with English (`'en'`). These chains are constructed in `\Wikibase\Lib\LanguageFallbackChainFactory\Wikibase\Lib\LanguageFallbackChainFactory`.
**Acceptance criteria**
* confirm the assumption that every term language fallback chain should end in English (`'en'`)
* consider adjusting the construction of that chain in `LanguageFallbackChainFactory`
** consider refactoring it first as it has a very high cyclomatic complexity and uses quite a lot of deprecated methods
* consider adjusting `TermLanguageFallbackChain` itself to always return at least `en`
* if it was decided not to take the actions above, consider adding tracking to the fallback behavior added in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/618703 so that we better understand the reasons for the chain being empty
**Notes:**
This has already caused the following incidents:
* {T259744}