Instead of raising AssertionError when users define custom axes
without explicit `<labelname xml:lang="en">ZZZZ</labelname>` element,
it's better to use the axis' name attribute, and treat it as "en"
language.
For example, if users generate the designspace from SuperPolator,
they can't edit the axis labelname attribute from the UI (or maybe
it's just me that couldn't figure out how to do it).