You're right Chouby, that define( 'wplang', '' ) doesn't conflict.
However, I reasure you that for me yoast wpseo strings only get to working by replicating them at a wpml-config.xml file.
I've tested it with and without the file and it works with but doesn't without.
This time the issue was at Archives meta titles, the other strings looked to be fine.
There was a strange behaviour too that caught my attention, when I placed the wpml-config.xml file at my theme's child theme folder, it didn't start to work imediatly, so after playing a bit with polylang definitions such as browser language detection, URL modifications options and permalink update because of the URL with or without /language/ and eventually re-save some of the posts tags and categories, my meta titles got working for each language.