When a label is not available in the language of the wiki that uses the property parser function a fallback should be applied. The label in an appropriate other language should be shown.
|Open||None||T124286 [Epic] Wikidata language support|
|Open||None||T76216 [Epic] Language fallback|
|Resolved||daniel||T76217 language fallback for the property parser function|
this is already done for variants. beyond that, it might get a bit weird and not quite as feasible in terms of caching.
languages like en-gb are not used as a content language anywhere afaik so wouldn't have the issue in those cases. there are 'languages' like simple english, in which case i think fallback should not apply unless the community explicitly decides they want such thing as some wiki-wide setting.