probably useful to know: If you append ?uselang=qqx (setting the UI language to a special code qqx), you’ll see all message identifiers. E.g. when visiting https://en.wikipedia.org/wiki/Main_Page?uselang=qqx you’d see the sidebar menu items are rendered using messages “contents”, “featuredcontent”, and “interaction”. (However, as Amir already noted, those are local customizations on enwiki, so you’d need to translate them locally as well, by asking a local sysop to put the translations in the MediaWiki namespace.)