Particularly annoying is that there's no way to do lists with attributes. Looks good:

  <user name="alice" group="wheel" />

Oh no:

  <user name="alice" groups="wheel admin sudoers" />

Or is it one of:

  <user name="alice" groups="wheel,admin,sudoers" />
  <user name="alice" groups="wheel:admin:sudoers" />
  <user name="alice" groups="wheel;admin;sudoers" />

Or give up and use elements:

  <user name="alice">
    <group>wheel</group>
    <group>admin</group>
    <group>sudoers</group>
  </user>

Hold on, should there be a container?

  <user name="alice">
    <groups>
      <group>wheel</group>
      <group>admin</group>
      <group>sudoers</group>
    </groups>
  </user>

XML really needs either richer attributes, or no attributes.

Replace with "Element" with "Class" and "Attribute" with "Property" in your examples.

It's your job to decide how the data should be structured, in any language.

I would do the following :

  <user name="alice">
    <groups>
      <group uid="wheel"/>
      <group uid="admin"/>
      <group uid="sudoers"/>
    </groups>
  </user>

Attributes are XML’s foot-gun.

I disagree. Back in the day we used attributes for everything that was key value and inner tags for anything with structure. We also formatted for clarity:

    <lunch
      env="outside"
      food="sandwiches"
      drink="cola"
    />

Compared with what we used to do, I look at attribute-less maven pom.xml with horror.

The decision of when and how to use attributes is the thing that is most often done wrong or clumsily in an XML file. That is my point.

What if it's to be used by french speaking software/people ?

    <lunch
      env="outside"
      envfr="dehors"
      food="sandwiches"
      foodfr="sandwichs"
      drink="cola"
    />

Or

    <lunch
      env="outside"
      food="sandwiches"
      drink="cola">
          <label type="env" language="fr">Dehors</label>
          <label type="env" language="de">Außenseite</label>
          <label type="env" language="en">Outside</label>
    </lunch>

Quite curious about it.

I guess this way:

  <lunch
    env="outside"
    food="sandwiches"
    drink="cola">
    <label xml:lang="fr">Dehors</label>
    <label xml:lang="de">Draußen</label>
    <label xml:lang="en">Outside</label>
  </lunch>

says[0]the w3.

[0]https://www.w3.org/TR/REC-xml/#sec-lang-tag

I see, thanks. (I also see you corrected my broken German ^^).

Putting l10n/i18n data inline is generally not a pleasant experience, and this problem persists regardless of what serialization format you are using. Either have separate data files per-locale that are merged with the defaults or store the localized strings in a central location (ala your usual gettext setup).