For be-x-old.wikipedia.org (lang. code: be-tarask), when a site-link is added for the Wikidata item, 'lang' attribute for site-link page title is set to 'be-x-old' which is wrong (it should be 'be-tarask').
As I see, it's either Repo not aware of the true wiki language, or it's a configuration concern and the language was set according to the domain (see Bug 9823).
To conclude, I think for now it's mostly a minor layout issue, but who knows... It's better to have real internal data.