The original plan was to use Jekyll collections to translate projects, but it turned out that the logic behind the collections is different from the posts.
It is not possible to combine pages from projects and en/project into one collection, so I had to use site.pages.

Now in _config.yml for projects and en/project it says layout: "project".

- scope:
    path: "en/projects"
  values:
    layout: "project"

- scope:
    path: "projects"
  values:
    layout: "project"

And you can get all projects for the current language like this:
{%- assign projects = site.pages | where: 'layout', 'project' | where: 'lang', page.lang -%}
Or for another language:
{%- assign projects = site.pages | where: 'layout', 'project' -%}
{%- assign tpages = projects | where: 'name', page.name | sort: 'lang' -%}