I want share my problem and solution.
Today i've discovered that my meta description and all other SEO string were not changing with the language of the page.
They were set correctly, polylang was able to get them with a manually test (ex: pll__(__('test var')).
So wpml-compat.php line 362 was doing all correctly.
I'm gone to check if the problem was SEO by Yoast and apparently i was a bit confused because testing class-frontend.php and checking get_wpseo_options() and metadesc() the array was in the default language and for no reason was the right language.. anyway...to make a long story short i've set in polylangs->settings->url modifications->"The language code, for example /en/, is added to all urls when using pretty permalinks." than i've re-saved in settings->permalinks and all was working good.
While i was finishing this post i've checked for the 1000 times the documentation of polylang and i've found this sentence "For example, it is recommended to use the second option when running Polylang together with WordPress SEO by Yoast."
In my humble opinion it's better add this thing to the FAQ:
I've installed SEO by Yoast but the custom strings are not working properly
Answer with the solution above.
I hope this will save someone. ^^
As usual sorry for my English.
I've added here this post because is similar to this thread and this other old thread