Currently if a question is tagged only with one of the wix tags, any code blocks in the question don't get syntax highlighting. Given that WiX is XML based, I think it would be helpful for the tag to trigger XML syntax highlighting.
You can force a specific set of language highlighting manually (doesn't go into the issue of WiX not being flagged as XML highlighting by default).
You should be able to set in the block the XML language
<!-- language: lang-xml --> to force the highlights. If you look at your specific example, the first couple of lines of your post would be:
<Component Id='XYZAppShortcutFr' Guid='XXXXXX'> <Condition>LANGUAGE="fr"</Condition> <Shortcut Id='ShortcutFr' Name='Démarrer $(var.ShortcutName)' Target='[INSTALLDIR]$(var.TargetEXE)' WorkingDirectory='INSTALLDIR' >