Please use either `as_xml_get_node_value` or `as_xml_get_node_value_raw`. You probably want the former and then check for NULL.

This is missing a `g_autofree`

Please don't mix declarations and statements.

nit: This could be one line with the one above it.

Unless this causes problems, making this a WARNING or possibly even INFO might be better. That allows some flexibility for writers of custom data (e.g. to have placeholders that are ignored deliberately).

=> the \`key\` attribute

Again, any literal should be in backticks so translators don't translate it by accident.