How will language fall back work, i.e. in which order should languages be used when a different language does not exist? How should a label in a different language be displayed so it is visually clear that this label is in a specific language? How does this tie-in with Bug T38426 ?
T40399: Multiple fallbacks kills server
T40439: Language factory uses too simplistic caching
T57998: Wikidata have to provide a syntax to show the properties in a language different from the wiki's content language
T39459: [Task] Don't try to add labels in non-existing languages: restrict to Language::isKnownLanguageTag