I managed to translate part of the contents by suppling in each page created with polylang the specific default_action in the shortcode as per documentation, still there are some untranslated and duplicated contents :
for the login/register widget the widget title and the user links will always stay at the defaul language, the form labels are translated correctly
The pages created with polylang and the default_action specified in the shortcode always output the current action name which can match the page title and is thus unecessary, yet the label is not enclosed in any html tag so it's not even possible to hide it via CSS
Regarding the themed profile page, if accesses with the default language at the page created by TML when not logged in will redirect to the login form, instead accessing a page with the default_action set to profile when not logged in will return the following error :
Warning: reset() expects parameter 1 to be array, null given in /home/simonere/public_html/wp-content/plugins/theme-my-login/modules/themed-profiles/themed-profiles.php on line 265
also the profile form will appear empty below the error