It is not ULS either. It's coming (or rather no language links are coming) from PHP side. It seems Wiktionary main pages usually manage their language links manually (the few ones I checked).
I think the cause is that, unlike in every other project, Wikidata interwikis are not enabled in the main namespace in Wiktionary (main space interlanguage links are provided by Cognate rather than Wikidata). The same issue is present on srwiktionary (except for that 50 interlanguage links are shown using the good old [[en:]] syntax, but WD has 162 pages apart from the Serbian one), but not on enwiktionary; slwiktionary and srwiktionary have their main pages in main namespace, enwiktionary has in project namespace. Either main page should be special-cased, or a parser function (opposite of noexternallanglinks) should be created to force showing the interwiki links in namespaces where it wouldn’t show up otherwise.