Querying content of a certain language code

[bnomei]

using the x-language header does not work nor can i set the language in the php kql() helper as far as i can tell.
https://getkirby.com/docs/guide/api/introduction#language

the forum suggests it might have worked before. maybe in v1?
https://forum.getkirby.com/t/kql-querying-pages-for-all-languages/20272

[bnomei]

this post here suggests that a language prop might have existed in v1.
#8 (comment)

const query = {
      query: `site.children.filterBy('uid', 'projekte').first`,
      language: this.$i18n.locale,
      select: {
        title: true,