Hi,
firstly, thanks for great plugin!
I use WordPress SEO by Yoast plugin within my multisite installation. When I activated Polylang and add second language I have problem with default translatable taxonomies (categories and tags) and custom taxonomies when I set it to be translatable. SEO values like Title or Description are not saved. When i look to my database, values are here but for some reason disappeared from inputs in administration and not used on web. I think there is problem within saving meta informations to database.
I use:
WordPress 3.9.1 Multisite (clean installation within /%postname%/ permalinks)
Polylang 1.5.2 (within folder settings like http://example.com/en/some-page/)
WordPress SEO by Yoast 1.5.3.3
Thanks for reaction and sorry for my english