Metadata with more than one language

I want to use more than one language in the creation of my Metadata. I was looking at the pdfx package documentation. And as I understood it, that would be possible. Based on the information in section 2.3, I built the following code.

RequirePackage{filecontents}
begin{filecontents*}{jobname.xmpdata}
    Title{Document Naamsep[en-US] Document name}
    Author{Name 1sep Name 2}
    Language{desep en-US}
    Subject{De samenvatting of korte beschrijving.sep[en-US] The abstract, or short description.}
    Keywords{trefwoordsep[en-US] keyword}
end{filecontents*}

This code is inserted before documentclass. The result is as follows,

Note that the description contains only the German description. I imagined that where I put a dark square it would be enabled and could change between languages. Was my guess wrong or is there something wrong with my code? I believe that my idea was wrong, I don’t think that where I signaled it means changing languages.

On the keywords, it is appearing en-US, I would like it to be showing only trefwoord and in advanced it was possible to see the keywords in English.

In the advanced options I have,

In the title and description the two options are showing only in German. And besides it is showing [x-default], how would I go about showing de?

Note: I am using usepackage[a-2b,mathxmp]{pdfx}[2018/12/22].

documentclass{article}
usepackage[a-2b]{pdfx}
begin{filecontents*}[overwrite]{jobname.xmpdata}
    Title{Document Naamsep[nl]Document Naamsep[en-US] Document name}
    Author{Name 1sep Name 2}
    Language{nlsep en-US}
    Subject{De samenvatting of korte beschrijving.sep[nl]De samenvatting of korte beschrijving.sep[en-US] The abstract, or short description.}
    Keywords{trefwoordsep[nl]trefwoordsep[en-US] keyword}
end{filecontents*}