This issue has been answered at Github here:
https://github.com/modxcms/revolution/issues/11240
The issue is that $modx->lexicon() uses the current cultureKey to load the string. So if you load a different lexicon language you also need to call lexicon() with that language like
$modx->lexicon->load('fr:core:about');
return $modx->lexicon('about_msg', array(), 'fr');