Purpose
=======
Add "Activate" button when selecting multiple languages, select multiple
languages when clicking on "Add languages" in settings.
Specifications
=============
`lang` variable in `base.language.install` changed from Selection to
Many2many to allow multiple languages being activated at once.
Hide globe icon for language fields from view mode (only visible when
editing). Remove state in base.language.install since it's no longer
need to keep track of the installation step.
Task-2662548
closesodoo/odoo#78287
Related: odoo/upgrade#2921
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>