Context:
In commit [00e6b87c9d16af1bfb3e63b9b857fbaad90e620a](https://github.com/odoo/odoo/commit/00e6b87c9d16af1bfb3e63b9b857fbaad90e620a) the deletion of
some code created a bug in the sign app where the language is set
in the html_data and the frontend translations were hence broken.
In the stable versions this commit was revert see PR https://github.com/odoo/odoo/pull/90757.

On master we make a cleaner fix and find a solution 
 solution in the sign module. By creating the get_frontend_session_info_sign 
we add the lang to the user_context and we don't need to extract it from the html attributes.
We also remove the deprecated xml line of the first fix commit https://github.com/odoo/enterprise/commit/7d59eb832d727522fc3b4639634c377e66ce3c59 that are now irelevant
as we don't use the html_data anymore.

A recent fix was merged concerning a close but different issue regarding translations in the sign
frontend https://github.com/odoo/odoo/commit/2b080ddbd8f332299b2343fa5ca0e0061b0ad554,
but this issue fixes the specific case of only one language (that is not ENG) activated. While here 
we solve the issue where there are multiple languages and the client that receives the email to sign
the document should have, when he clicks on the document link, the translations in the language set 
in its preferences. 

opw-2777044