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).