The language links of the Beta-Cluster are to the production wikis (see T69931) which is incompatible with `Link.langlinkUnsafe` as that requires to be in the same family.
Now for example https://gerrit.wikimedia.org/r/174827 tries to fix certain issues with our `QueryGenerator` and there I remove the `expectedFailure` from `tests.site_tests.TestPagePreloading.test_preload_langlinks_normal` and now get an error that [[https://travis-ci.org/xZise/pywikibot-core/jobs/79537609#L926|`bar` is not in the family `wpbeta`]].
Now I'm wondering whether we should deprecate that method and only use `APISite.interwiki` to actually get the site related to the given language prefix. Alternatively we could avoid that in our tests using something similar to deb64e80 but it would only fix the patch and not really this problem.