Problem/Motivation
D7 introduced great feature: language fallback that was used for entity translation. Unfortunately this feature was used only for entity translation. A contributed module was created to add language fallback features to the string translations too.
Unfortunately it seems that this inconsistency in translation mechanisms is now also present in D8. The core string translation ignores language fallback features that are also provided by the core.
Proposed resolution
It all boils down to the LocaleLookup::resolveCacheMiss function that in case of missing translation should check fallback languages offered by LanguageManager::getFallbackCandidates() function.
Remaining tasks
Create a patch.
User interface changes
No user interface changes.
API changes
No API changes beside the fact that finally language fallback will work also for string translation.