It would be useful to be able to use Lua to fetch a description given an input language, without having to fetch and parse the whole item. Ideally, this would also allow an option to use the fallback languages or not.
According to the documentation at:
we currently have mw.wikibase.getLabelByLang and mw.wikibase.getDescription - but not mw.wikibase.getDescriptionByLang.
Note that the "WithLang" options aren't relevant here, as they just return the defined language, they don't let you select the language to use.