On: XML is almost always misused

[progval]

No, GP didn't. They use tag names as keys.

But you answer your question, yes this format can be expressed with XML-Schema:

  <xsd:element name="dict">
    <xsd:complexType>
      <xsd:sequence minOccurs="0" maxOccurs="unbounded">
        <xsd:element name="key" type="xsd:string" />
        <xsd:choice>
          <xsd:element name="string" type="xsd:string" />
          <xsd:element name="integer" type="xsd:integer" />
          <!-- etc. -->
        </xsd:choice>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>

[spiralx → progval]

Or in RELAX-NG compact :)

    grammar {
      start = Dict
      Dict = element dict { DictItem* }
      DictItem =
        element key { text },
        (element string { text } | element number { xsd:integer })
    }