User-specific fallback (using babel boxes, etc) is currently used for listings like RecentChanges. It cannot be used for page content, since it would poison (or disable) the parser cache.
It is currently also used in "some" other places, at least in the GetEntities API module (with the languagefallback parameter set). This can be confusing when trying to use the API to interact with page content (the way Gadgets often do).
We should survey when and where we use user specific fallback (via LanguageFallbackChainFactory::fromContext, and simmilar methods). We should then decide in which places it is sensible to use this. Since we can't use it everywhere, perhaps we should stop using it at all, to avoid confusion.